CheatSheet.lhs

\documentclass[11pt]{article}
%include lhs2TeX.fmt
\usepackage[T1]{fontenc}
\usepackage[sc]{mathpazo}
\linespread{1.05}
\usepackage{helvet}

\usepackage{multicol}
\usepackage{float}
\usepackage[landscape, top=0.2in, bottom=1in, left=0.2in, right=0.2in, dvips]{geometry}
\usepackage{verbatim}
\usepackage{fancyhdr}
\usepackage{paralist}

\usepackage{hyperref}
\usepackage[all]{hypcap} % Must be after hyperref
% \usepackage{setspace}
\hypersetup{colorlinks}

\pagestyle{fancy}
\fancyhf{}
\lfoot{\copyright\ 2010 Justin Bailey.}
\cfoot{\thepage}
\rfoot{\href{mailto:jgbailey@@codeslower.com}{\tt jgbailey@@codeslower.com}}
\renewcommand\footrulewidth{0.4pt}

\makeatletter
% Copied from article.cls; second-to-last parameter changed to -\parindent.
\renewcommand\subsubsection{\@@startsection{subsubsection}{3}{\z@@}%
  {-3.25ex \@@plus -1ex \@@minus -.2ex}%
  {-\parindent}%
  {\normalfont\normalsize\bfseries}}
\makeatother

\newcommand{\hd}[1]{\section*{\textsf{#1}}}
\newcommand{\shd}[1]{\subsection*{\textsf{#1}}}
\newcommand{\sshd}[1]{\subsubsection*{\textsf{#1}}}
\setlength{\columnsep}{18.0pt}
\setlength{\columnseprule}{0.4pt}
\begin{document}
% \doublespacing
\begin{multicols}{3}
\section*{\textsf{\LARGE Haskell Cheat Sheet\normalsize}}\label{preamble}

This cheat sheet lays out the fundamental elements of the Haskell language:
syntax, keywords and other elements. It is presented as both an executable
Haskell file and a printable document. Load the source into your favorite
interpreter to play with code samples shown.

\begin{comment}

> {-# LANGUAGE MultiParamTypeClasses,NPlusKPatterns, DatatypeContexts #-}
>
> module CheatSheet where
>
> import Data.Char (isUpper, isLower, toUpper, toLower, isSpace, GeneralCategory(..))
> import System.IO (readFile)
> import System.Directory (doesFileExist)
> import qualified Data.Set as Set
> import qualified Data.Char as Char

\end{comment}

\hd{Basic Syntax}\label{syntax}

\shd{Comments}\label{comments}

  A single line comment starts with `@--@' and extends to the end of the line.
  Multi-line comments start with '@{-@' and extend to '@-}@'. Comments can be
  nested.

  Comments above function definitions should start with `@{- |@' and those next
  to parameter types with `@-- ^@' for compatibility with
  Haddock, a system for documenting
  Haskell code.

\shd{Reserved Words}\label{reserved-words}

  The following words are reserved in Haskell. It is a syntax error to give a
  variable or a function one of these names.

  \setlength{\columnsep}{10.0pt}
  \setlength{\columnseprule}{0.0pt}
  \begin{multicols}{3}
    \begin{compactitem}
      \item @case@
      \item @class@
      \item @data@
      \item @deriving@
      \item @do@
      \item @else@
      \item @if@
      \item @import@
      \item @in@
      \item @infix@
      \item @infixl@
      \item @infixr@
      \item @instance@
      \item @let@
      \item @of@
      \item @module@
      \item @newtype@
      \item @then@
      \item @type@
      \item @where@
    \end{compactitem}
  \end{multicols}
  \setlength{\columnsep}{18.0pt}
  \setlength{\columnseprule}{0.4pt}

\shd{Strings}\label{strings}

  \begin{compactitem}
  \item @"abc"@ -- Unicode string, sugar for @['a','b','c']@.
  \item @'a'@ -- Single character.
  \end{compactitem}

  \sshd{Multi-line Strings}\label{multi-line-strings}

  Normally, it is a syntax error if a string has any newline characters.
  That is, this is a syntax error:

< string1 = "My long
< string."

  Backslashes (`@\@') can ``escape'' a newline:

> string1 = "My long \
> \string."

  The area between the backslashes is ignored. Newlines \emph{in} the
  string must be represented explicitly:

> string2 = "My long \n\
> \string."

  That is, @string1@ evaluates to:

< My long string.

  While @string2@ evaluates to:

< My long
< string.

  \sshd{Escape Codes} The following escape codes can be used in characters or strings:
  \begin{compactitem}
    \item @\n@, @\r@, @\f@, etc. -- The standard codes for newline,
      carriage return, form feed, etc. are supported.
    \item @\72@, @\x48@, @\o110@ -- A character with the value 72 in
      decimal, hex and octal, respectively.
    \item @\&@ -- A ``null'' escape character which allows numeric
      escape codes next to numeric literals. For example, @\x2C4@ is
      $\wedge$ (in Unicode) while @\x2C\&4@ is @,4@. This sequence
      cannot be used in character literals.
  \end{compactitem}


\shd{Numbers}\label{numbers}

  \begin{compactitem}
  \item @1@ -- Integer or floating point value.
  \item @1.0, 1e10@ -- Floating point value.
  \item @0o1, 0O1@ -- Octal value.
  \item @0x1, 0X1@ -- Hexadecimal value.
 \item  @-1@ -- Negative number; the minus sign (``@-@'') cannot be separated from the number.
  \end{compactitem}

\shd{Enumerations}\label{enumerations}

  \begin{compactitem}
  \item @[1..10]@ -- List of numbers -- \texttt{1, 2, {\ensuremath\mathellipsis}, 10}.
  \item @[100..]@ -- Infinite list of numbers -- \texttt{100, 101, 102, {\ensuremath\mathellipsis}\ }.
  \item @[110..100]@ -- Empty list, but @[110, 109 .. 100]@ will give a list from 110 to 100.
  \item @[0, -1 ..]@ -- Negative integers.
  \item @[-110..-100]@ -- Syntax error; need @[-110.. -100]@ for negatives.
  \item @[1,3..99], [-1,3..99]@ -- List from 1 to 99 by 2, -1 to 99 by 4.
  \end{compactitem}

  \noindent In fact, any value which is in the @Enum@ class can be used:

  \begin{compactitem}
  \item @['a' .. 'z']@ -- List of characters -- \texttt{a, b, {\ensuremath\mathellipsis}, z}.
  \item @['z', 'y' .. 'a']@ -- \texttt{z, y, x, {\ensuremath\mathellipsis}, a}.
  \item @[1.0, 1.5 .. 2]@ -- @[1.0,1.5,2.0]@.
  \item @[UppercaseLetter ..]@ -- List of @GeneralCategory@ values (from @Data.Char@).
  \end{compactitem}

\shd{Lists \& Tuples}\label{lists-tuples}

  \begin{compactitem}
  \item @[]@ -- Empty list.
  \item @[1,2,3]@ -- List of three numbers.
  \item @1 : 2 : 3 : []@ -- Alternate way to write lists using ``cons'' (@:@) and ``nil'' (@[]@).
  \item @"abc"@ -- List of three characters (strings are lists).
  \item @'a' : 'b' : 'c' : []@ -- List of characters (same as @"abc"@).
  \item @(1,"a")@ -- 2-element tuple of a number and a string.
  \item @(head, tail, 3, 'a')@ -- 4-element tuple of two functions, a number and a character.
  \end{compactitem}

\shd{``Layout'' rule, braces and semi-colons.}\label{layout}

 Haskell can be written using braces and semi-colons, just like C. However, no
 one does. Instead, the ``layout'' rule is used, where spaces represent scope.
 The general rule is: always indent. When the compiler complains, indent more.

  \sshd{Braces and semi-colons}\label{braces-semicolons}

  Semi-colons terminate an expression, and braces represent scope. They can be
  used after several keywords: @where@, @let@, @do@ and @of@. They cannot be
  used when defining a function body. For example, the below will not compile.

<    square2 x = { x * x; }

  However, this will work fine:

> square2 x = result
>     where { result = x * x; }

  \sshd{Function Definition}\label{layout-function-definition}

  Indent the body at least one space from the function name:

< square x  =
<   x * x

  Unless a @where@ clause is present. In that case, indent the where clause at
  least one space from the function name and any function bodies at least one
  space from the @where@ keyword:

<  square x =
<      x2
<    where x2 =
<      x * x

  \sshd{Let}\label{layout-let}

  Indent the body of the let at least one space from the first definition in the
  @let@. If @let@ appears on its own line, the body of any definition must
  appear in the column after the let:

<  square x =
<    let x2 =
<          x * x
<    in x2

  As can be seen above, the @in@ keyword must also be in the same column as
  @let@. Finally, when multiple definitions are given, all identifiers must
  appear in the same column.

\hd{Declarations, Etc.}\label{declarations}

  The following section details rules on function declarations, list
  comprehensions, and other areas of the language.

\shd{Function Definition}\label{function-definition}

  Functions are defined by declaring their name, any arguments, and an equals
  sign:

> square x = x * x

  \emph{All} functions names must start with a lowercase letter or ``@_@''. It
  is a syntax error otherwise.

  \sshd{Pattern Matching}\label{pattern-matching}

  Multiple ``clauses'' of a function can be defined by ``pattern-matching'' on
  the values of arguments. Here, the @agree@ function has four separate
  cases:

> -- Matches when the string "y" is given.
> agree1 "y" = "Great!"
> -- Matches when the string "n" is given.
> agree1 "n" = "Too bad."
> -- Matches when string beginning
> -- with 'y' given.
> agree1 ('y':_) = "YAHOO!"
> -- Matches for any other value given.
> agree1 _ = "SO SAD."

  Note that the `@_@' character is a wildcard and matches any value.

  Pattern matching can extend to nested values. Assuming this data declaration:

< data Bar = Bil (Maybe Int) | Baz

  \noindent and recalling the \hyperref[maybe]{definition of @Maybe@} from
  page~\pageref{maybe} we can match on nested @Maybe@ values when @Bil@ is
  present:

< f (Bil (Just _)) = ...
< f (Bil Nothing) = ...
< f Baz = ...

  Pattern-matching also allows values to be assigned to variables. For example,
  this function determines if the string given is empty or not. If not, the
  value bound to @str@ is converted to lower case:

> toLowerStr [] = []
> toLowerStr str = map toLower str

  Note that @str@ above is similar to @_@ in that it will match anything; the
  only difference is that the value matched is also given a name.

  \sshd{{\ensuremath $n + k$} Patterns}\label{plus-patterns}

  This (sometimes controversial) pattern-matching facility makes it easy to match
  certain kinds of numeric expressions. The idea is to define a base case (the
  ``$n$'' portion) with a constant number for matching, and then to define other
  matches (the ``$k$'' portion) as additives to the base case. Here is a rather
  inefficient way of testing if a number is even or not:

> isEven 0 = True
> isEven 1 = False
> isEven (n + 2) = isEven n

  \sshd{Argument Capture}\label{argument-capture}

  Argument capture is useful for pattern-matching a value \emph{and} using it,
  without declaring an extra variable. Use an `|@|' symbol in between the
  pattern to match and the variable to bind the value to. This facility is
  used below to bind the head of the list in @l@ for display, while also
  binding the entire list to @ls@ in order to compute its length:

> len ls@(l:_) = "List starts with " ++
>   show l ++ " and is " ++
>   show (length ls) ++ " items long."
> len [] = "List is empty!"

  \sshd{Guards}\label{function-guards}

  Boolean functions can be used as ``guards'' in function definitions along with
  pattern matching. An example without pattern matching:

> which n
>   | n == 0 = "zero!"
>   | even n = "even!"
>   | otherwise = "odd!"

    Notice @otherwise@ -- it always evaluates to @True@ and can be used to specify
    a ``default'' branch.

    Guards can be used with patterns. Here is a function that determines if the
    first character in a string is upper or lower case:

> what [] = "empty string!"
> what (c:_)
>   | isUpper c = "upper case!"
>   | isLower c = "lower case"
>   | otherwise = "not a letter!"

  \sshd{Matching \& Guard Order}\label{function-matching-order}

  Pattern-matching proceeds in top to bottom order. Similarly, guard expressions
  are tested from top to bottom. For example, neither of these functions would
  be very interesting:

> allEmpty _ = False
> allEmpty [] = True
>
> alwaysEven n
>   | otherwise = False
>   | n `div` 2 == 0 = True

  \sshd{Record Syntax}\label{matching-record-syntax}

  Normally pattern matching occurs based on the position of arguments in the
  value being matched. Types declared with record syntax, however, can match
  based on those record names. Given this data type:

> data Color = C { red
>   , green
>   , blue :: Int }

\begin{comment}

>   deriving (Show, Eq)

\end{comment}

  \noindent we can match on @green@ only:

> isGreenZero (C { green = 0 }) = True
> isGreenZero _ = False

  Argument capture is possible with this syntax, although it gets clunky.
  Continuing the above, we now define a @Pixel@ type and a function to replace
  values with non-zero @green@ components with all black:

> data Pixel = P Color

\begin{comment}

>   deriving (Show, Eq)

\end{comment}

> -- Color value untouched if green is 0
> setGreen (P col@(C { green = 0 })) = P col
> setGreen _ = P (C 0 0 0)

  \sshd{Lazy Patterns}\label{lazy-patterns}

  This syntax, also known as \emph{irrefutable} patterns, allows pattern matches
  which always succeed. That means any clause using the pattern will succeed,
  but if it tries to actually use the matched value an error may occur. This is
  generally useful when an action should be taken on the \emph{type} of a
  particular value, even if the value isn't present.

  For example, define a class for default values:

> class Def a where
>   defValue :: a -> a

  The idea is you give @defValue@ a value of the right type and it gives you
  back a default value for that type. Defining instances for basic types is
  easy:

> instance Def Bool where
>   defValue _ = False
>
> instance Def Char where
>   defValue _ = ' '

  @Maybe@ is a littler trickier, because we want to get a default value for the
  type, but the constructor might be @Nothing@. The following definition would
  work, but it's not optimal since we get @Nothing@ when @Nothing@ is passed in.

< instance Def a => Def (Maybe a) where
<   defValue (Just x) = Just (defValue x)
<   defValue Nothing = Nothing

  We'd rather get a {\tt Just (\rm\emph{default value}\tt)\rm} back instead.
  Here is where a lazy pattern saves us -- we can pretend that we've matched
  @Just x@ and use that to get a default value, even if @Nothing@ is given:

> instance Def a => Def (Maybe a) where
>   defValue ~(Just x) = Just (defValue x)

  As long as the value @x@ is not actually evaluated, we're safe. None of the
  base types need to look at @x@ (see the ``@_@'' matches they use), so things
  will work just fine.

  One wrinkle with the above is that we must provide type annotations in the
  interpreter or the code when using a @Nothing@ constructor. @Nothing@ has type
  @Maybe a@ but, if not enough other information is available, Haskell must be
  told what @a@ is. Some example default values:

> -- Return "Just False"
> defMB = defValue (Nothing :: Maybe Bool)
> -- Return "Just ' '"
> defMC = defValue (Nothing :: Maybe Char)

\shd{List Comprehensions}\label{list-comprehensions}

  A list comprehension consists of four types of elements: \emph{generators},
  \emph{guards}, \emph{local bindings}, and \emph{targets}. A list comprehension
  creates a list of target values based on the generators and guards given. This
  comprehension generates all squares:

> squares = [x * x | x <- [1..]]

  @x <- [1..]@ generates a list of all @Integer@ values and puts them in @x@,
  one by one. @x * x@ creates each element of the list by multiplying @x@ by
  itself.

  Guards allow certain elements to be excluded. The following shows how divisors
  for a given number (excluding itself) can be calculated. Notice how @d@ is
  used in both the guard and target expression.

> divisors n =
>   [d | d <- [1..(n `div` 2)]
>      , n `mod` d == 0]

  Local bindings provide new definitions for use in the generated expression or
  subsequent generators and guards. Below, @z@ is used to represent the minimum
  of @a@ and @b@:

> strange = [(a,z) | a <-[1..3]
>                  , b <-[1..3]
>                  , c <- [1..3]
>                  , let z = min a b
>                  , z < c ]

  Comprehensions are not limited to numbers. Any list will do. All upper case
  letters can be generated:

> ups =
>   [c | c <- [minBound .. maxBound]
>      , isUpper c]

  Or, to find all occurrences of a particular break value @br@ in a list @word@
  (indexing from 0):

> idxs word br =
>   [i | (i, c) <- zip [0..] word
>       , c == br]

  A unique feature of list comprehensions is that pattern matching failures do
  not cause an error; they are just excluded from the resulting list.

\shd{Operators}\label{operators}

  There are very few predefined ``operators'' in Haskell---most that appear
  predefined are actually syntax (e.g., ``@=@''). Instead, operators are simply
  functions that take two arguments and have special syntactic support. Any
  so-called operator can be applied as a prefix function using parentheses:

< 3 + 4 == (+) 3 4

  To define a new operator, simply define it as a normal function, except the
  operator appears between the two arguments. Here's one which inserts a
  comma between two strings and ensures no extra spaces appear:

> first ## last =
>   let trim s = dropWhile isSpace
>         (reverse (dropWhile isSpace
>           (reverse s)))
>   in trim last ++ ", " ++ trim first

< > "  Haskell " ## " Curry "
< Curry, Haskell

  Of course, full pattern matching, guards, etc. are available in this form.
  Type signatures are a bit different, though. The operator ``name'' must appear
  in parentheses:

> (##) :: String -> String -> String

  Allowable symbols which can be used to define operators are:

< # $ % & * + . / < = > ? @ \ ^ | - ~

  However, there are several ``operators'' which cannot be redefined. They are:
  @<-@, @->@ and @=@. The last, @=@, cannot be redefined by itself, but can be
  used as part of multi-character operator. The ``bind'' function, @>>=@, is one
  example.

  \sshd{Precedence \& Associativity}\label{fixity}

  The precedence and associativity, collectively called \emph{fixity}, of any
  operator can be set through the @infix@, @infixr@ and @infixl@ keywords. These
  can be applied both to top-level functions and to local definitions. The
  syntax is:

\bigskip
  \textbraceleft\texttt{infix} || \texttt{infixr} || \texttt{infixl}\textbraceright\ \emph{precedence op}
\bigskip

  \noindent where \emph{precedence} varies from 0 to 9. \emph{Op} can actually
  be any function which takes two arguments (i.e., any binary operation).
  Whether the operator is left or right associative is specified by @infixl@ or
  @infixr@, respectively. Such @infix@ declarations have no associativity.

  Precedence and associativity make many of the rules of arithmetic work ``as
  expected.'' For example, consider these minor updates to the precedence of
  addition and multiplication:

> infixl 8 `plus1`
> plus1 a b = a + b
> infixl 7 `mult1`
> mult1 a b = a * b

  The results are surprising:

< > 2 + 3 * 5
< 17
< > 2 `plus1` 3 `mult1` 5
< 25

  Reversing associativity also has interesting effects. Redefining division as
  right associative:

> infixr 7 `div1`
> div1 a b = a / b

  We get interesting results:

< > 20 / 2 / 2
< 5.0
< > 20 `div1` 2 `div1` 2
< 20.0

\shd{Currying}\label{currying}

 In Haskell, functions do not have to get all of their arguments at once. For
 example, consider the @convertOnly@ function, which only converts certain
 elements of string depending on a test:

> convertOnly test change str =
>     map (\c -> if test c
>                 then change c
>                 else c) str

 Using @convertOnly@, we can write the @l33t@ function which converts certain
 letters to numbers:

> l33t = convertOnly isL33t toL33t
>   where
>     isL33t 'o' = True
>     isL33t 'a' = True
>     -- etc.
>     isL33t _ = False
>     toL33t 'o' = '0'
>     toL33t 'a' = '4'
>     -- etc.
>     toL33t c = c

 Notice that @l33t@ has no arguments specified. Also, the final argument to
 @convertOnly@ is not given. However, the type signature of @l33t@ tells the
 whole story:

< l33t :: String -> String

 That is, @l33t@ takes a string and produces a string. It is a ``constant'', in
 the sense that @l33t@ always returns a value that is a function which takes a
 string and produces a string. @l33t@ returns a ``curried'' form of
 @convertOnly@, where only two of its three arguments have been supplied.

 This can be taken further. Say we want to write a function which only changes
 upper case letters. We know the test to apply, @isUpper@, but we don't want to
 specify the conversion. That function can be written as:

> convertUpper = convertOnly isUpper

 which has the type signature:

< convertUpper :: (Char -> Char)
<   -> String -> String

 That is, @convertUpper@ can take two arguments. The first is the conversion
 function which converts individual characters and the second is the string to
 be converted.

 A curried form of any function which takes multiple arguments can be created.
 One way to think of this is that each ``arrow'' in the function's signature
 represents a new function which can be created by supplying one more argument.

 \sshd{Sections}\label{sections}

 Operators are functions, and they can be curried like any other. For example, a
 curried version of ``@+@'' can be written as:

< add10 = (+) 10

 However, this can be unwieldy and hard to read. ``Sections'' are curried
 operators, using parentheses. Here is @add10@ using sections:

> add10 = (10 +)

 The supplied argument can be on the right or left, which indicates what
 position it should take. This is important for operations such as
 concatenation:

> onLeft str = (++ str)
> onRight str = (str ++)

 Which produces quite different results:

< > onLeft "foo" "bar"
< "barfoo"
< > onRight "foo" "bar"
< "foobar"

\shd{``Updating'' values and record syntax}\label{updating}

  Haskell is a pure language and, as such, has no mutable state. That is, once a
  value is set it never changes. ``Updating'' is really a copy operation, with
  new values in the fields that ``changed.'' For example, using the @Color@ type
  defined earlier, we can write a function that sets the @green@ field to zero
  easily:

> noGreen1 (C r _ b) = C r 0 b

  The above is a bit verbose and can be rewritten using record syntax. This kind
  of ``update'' only sets values for the field(s) specified and copies the rest:

> noGreen2 c = c { green = 0 }

  Here we capture the @Color@ value in @c@ and return a new @Color@ value.  That
  value happens to have the same value for @red@ and @blue@ as @c@ and it's
  @green@ component is 0. We can combine this with pattern matching to set the
  @green@ and @blue@ fields to equal the @red@ field:

> makeGrey c@(C { red = r }) =
>   c { green = r, blue = r }

  Notice we must use argument capture (``|c@|'') to get the @Color@ value and
  pattern matching with record syntax (``|C { red = r}|'') to get the inner
  @red@ field.

\shd{Anonymous Functions}\label{anonymous-functions}

  An anonymous function (i.e., a \emph{lambda expression} or \emph{lambda} for
  short), is a function without a name. They can be defined at any time like so:

< \c -> (c, c)

  which defines a function that takes an argument and returns a tuple
  containing that argument in both positions. They are useful for simple
  functions which don't need a name. The following determines if a string
  consists only of mixed case letters and whitespace.

> mixedCase str =
>   all (\c -> isSpace c ||
>              isLower c ||
>              isUpper c) str

  Of course, lambdas can be the returned from functions too. This classic
  returns a function which will then multiply its argument by the one originally
  given:

> multBy n = \m -> n * m

  For example:

< > let mult10 = multBy 10
< > mult10 10
< 100

\shd{Type Signatures}\label{type-signatures}

  Haskell supports full type inference, meaning in most cases no types have to
  be written down. Type signatures are still useful for at least two reasons.

  \begin{description}
  \item{\emph{Documentation}}---Even if the compiler can figure out the types
  of your functions, other programmers or even yourself might not be able to
  later. Writing the type signatures on all top-level functions is considered
  very good form.

  \item{\emph{Specialization}}---Typeclasses allow functions with overloading.
  For example, a function to negate any list of numbers has the signature:

< negateAll :: Num a => [a] -> [a]

  However, for efficiency or other reasons you may only want to allow @Int@
  types. You would accomplish that with a type signature:

< negateAll :: [Int] -> [Int]
  \end{description}

  Type signatures can appear on top-level functions and nested @let@ or @where@
  definitions. Generally this is useful for documentation, although in some
  cases they are needed to prevent polymorphism. A type signature is first the
  name of the item which will be typed, followed by a @::@, followed by the
  types. An example of this has already been seen above.

  Type signatures do not need to appear directly above their implementation.
  They can be specified anywhere in the containing module (yes, even below!).
  Multiple items with the same signature can also be defined together:

> pos, neg :: Int -> Int

< ...

> pos x | x < 0 = negate x
>       | otherwise = x
>
> neg y | y > 0 = negate y
>       | otherwise = y

  \sshd{Type Annotations}\label{type-annotations}

  Sometimes Haskell cannot determine what type is meant. The classic
  demonstration of this is the so-called ``@show . read@'' problem:

< canParseInt x = show (read x)

  Haskell cannot compile that function because it does not know the type of @read x@.
  We must limit the type through an annotation:

> canParseInt x = show (read x :: Int)

  Annotations have the same syntax as type signatures, but may adorn
  any expression. Note that the annotation above is on the expression
  @read x@, not on the variable @x@. Only function application (e.g.,
  @read x@) binds tighter than annotations. If that was not the case,
  the above would need to be written @(read x) :: Int@.

\shd{Unit}\label{unit}

  @()@ -- ``unit'' type and ``unit'' value. The value and type that represents
  no useful information.

\hd{Keywords}\label{keywords}

  Haskell keywords are listed below, in alphabetical order.

\shd{Case}\label{case}

  @case@ is similar to a @switch@ statement in C\# or Java, but can match a
  pattern: the shape of the value being inspected.  Consider a simple data type:

> data Choices = First String | Second |
>   Third | Fourth

\begin{comment}

>   deriving (Show, Eq)

\end{comment}

  \noindent @case@ can be used to determine which choice was given:

> whichChoice ch =
>   case ch of
>     First _ -> "1st!"
>     Second -> "2nd!"
>     _ -> "Something else."

  As with pattern-matching in function definitions, the `@_@' token is a
  ``wildcard'' matching any value.

  \sshd{Nesting \& Capture}\label{nesting-capture}

  Nested matching and binding are also allowed. For example, here is the definition
of the @Maybe@ type:

< data Maybe a = Just a | Nothing
\label{maybe}

  Using @Maybe@ we can determine if any choice was given using a nested match:

> anyChoice1 ch =
>   case ch of
>     Nothing -> "No choice!"
>     Just (First _) -> "First!"
>     Just Second -> "Second!"
>     _ -> "Something else."

  Binding can be used to manipulate the value matched:

> anyChoice2 ch =
>   case ch of
>     Nothing -> "No choice!"
>     Just score@(First "gold") ->
>       "First with gold!"
>     Just score@(First _) ->
>       "First with something else: "
>         ++ show score
>     _ -> "Not first."

  \sshd{Matching Order}\label{case-matching-order}

  Matching proceeds from top to bottom. If @anyChoice1@ is reordered as follows,
  the first pattern will always succeed:

> anyChoice3 ch =
>   case ch of
>     _ -> "Something else."
>     Nothing -> "No choice!"
>     Just (First _) -> "First!"
>     Just Second -> "Second!"

  \sshd{Guards}\label{case-guards}

  Guards, or conditional matches, can be used in cases just like function
  definitions. The only difference is the use of the @->@ instead of @=@. Here
  is a simple function which does a case-insensitive string match:

> strcmp s1 s2 = case (s1, s2) of
>   ([], []) -> True
>   (s1:ss1, s2:ss2)
>     | toUpper s1 == toUpper s2 ->
>         strcmp ss1 ss2
>     | otherwise -> False
>   _ -> False

\shd{Class}\label{class}

  A Haskell function is defined to work on a certain type or set of types and
  cannot be defined more than once. Most languages support the idea of
  ``overloading'', where a function can have different behavior depending on the
  type of its arguments. Haskell accomplishes overloading through @class@ and
  @instance@ declarations. A @class@ defines one or more functions that can be
  applied to any types which are members (i.e., instances) of that class. A
  class is analogous to an interface in Java or C\#, and instances to a concrete
  implementation of the interface.

  A class must be declared with one or more type variables. Technically, Haskell
  98 only allows one type variable, but most implementations of Haskell support
  so-called \emph{multi-parameter type classes}, which allow more than one type
  variable.

  We can define a class which supplies a flavor for a given type:

> class Flavor a where
>   flavor :: a -> String

  Notice that the declaration only gives the type signature of the function---no
  implementation is given here (with some exceptions, see
  \hyperref[defaults]{``Defaults''} on page~\pageref{defaults}). Continuing, we
  can define several instances:

> instance Flavor Bool where
>   flavor _ = "sweet"
>
> instance Flavor Char where
>   flavor _ = "sour"

  Evaluating @flavor True@ gives:

< > flavor True
< "sweet"

  While @flavor 'x'@ gives:

< > flavor 'x'
< "sour"

\sshd{Defaults}\label{defaults}

  Default implementations can be given for functions in a class. These are
  useful when certain functions can be defined in terms of others in the class.
  A default is defined by giving a body to one of the member functions. The
  canonical example is @Eq@, which defines @/=@ (not equal) in terms of @==@\ :

< class Eq a where
<   (==) :: a -> a -> Bool
<   (/=) :: a -> a -> Bool
<   (/=) a b = not (a == b)

  Recursive definitions can be created. Continuing the @Eq@ example,
  @==@ can be defined in terms of @/=@:

<   (==) a b = not (a /= b)

  However, if instances do not provide enough concrete implementations
  of member functions then any program using those instances will loop.

\shd{Data}\label{data}

  So-called \emph{algebraic data types} can be declared as follows:

> data MyType = MyValue1 | MyValue2

\begin{comment}

>   deriving (Show, Eq)

\end{comment}

  @MyType@ is the type's \emph{name}. @MyValue1@ and @MyValue2@ are \emph{values}
  of the type and are called \emph{constructors}. Multiple constructors are
  separated with the `@|@' character. Note that type and constructor names
  \emph{must} start with a capital letter. It is a syntax error otherwise.

  \sshd{Constructors with Arguments}\label{constructors-with-arguments}

  The type above is not very interesting except as an enumeration. Constructors
  that take arguments can be declared, allowing more information to be stored:

> data Point = TwoD Int Int
>   | ThreeD Int Int Int

  Notice that the arguments for each constructor are \emph{type} names, not
  constructors. That means this kind of declaration is illegal:

< data Poly = Triangle TwoD TwoD TwoD

  instead, the @Point@ type must be used:

> data Poly = Triangle Point Point Point

  \sshd{Type and Constructor Names}\label{type-punning}

  Type and constructor names can be the same, because they will never be used in
  a place that would cause confusion. For example:

> data User = User String | Admin String

  which declares a type named @User@ with two constructors, @User@ and @Admin@.
  Using this type in a function makes the difference clear:

> whatUser (User _) = "normal user."
> whatUser (Admin _) = "admin user."

  Some literature refers to this practice as \emph{type punning}.

  \sshd{Type Variables}\label{type-variables}

  Declaring so-called \emph{polymorphic} data types is as easy as adding type
  variables in the declaration:

> data Slot1 a = Slot1 a | Empty1

  This declares a type @Slot1@ with two constructors, @Slot1@ and @Empty1@. The
  @Slot1@ constructor can take an argument of \emph{any} type, which is
  represented by the type variable @a@ above.

  We can also mix type variables and specific types in constructors:

> data Slot2 a = Slot2 a Int | Empty2

  Above, the @Slot2@ constructor can take a value of any type and an @Int@
  value.

  \sshd{Record Syntax}\label{record-syntax}

  Constructor arguments can be declared either positionally, as above, or using
  record syntax, which gives a name to each argument. For example, here we
  declare a @Contact@ type with names for appropriate arguments:

> data Contact = Contact { ctName :: String
>       , ctEmail :: String
>       , ctPhone :: String }

  These names are referred to as \emph{selector} or \emph{accessor} functions
  and are just that, functions. They must start with a lowercase letter or
  underscore and cannot have the same name as another function in scope. Thus
  the ``@ct@'' prefix on each above. Multiple constructors (of the same type)
  can use the same accessor function for values of the same type, but that can
  be dangerous if the accessor is not used by all constructors. Consider this
  rather contrived example:

> data Con = Con { conValue :: String }
>   | Uncon { conValue :: String }
>   | Noncon
>
> whichCon con = "convalue is " ++
>   conValue con

  If @whichCon@ is called with a @Noncon@ value, a runtime error will occur.

  Finally, as explained elsewhere, these names can be used for pattern matching,
  argument capture and ``updating.''

  \sshd{Deriving}\label{deriving}

  Many types have common operations which are tedious to define yet necessary,
  such as the ability to convert to and from strings, compare for equality, or
  order in a sequence. These capabilities are defined as typeclasses in Haskell.

  Because seven of these operations are so common, Haskell provides the
  @deriving@ keyword which will automatically implement the typeclass on the
  associated type. The seven supported typeclasses are: @Eq@, @Read@, @Show@,
  @Ord@, @Enum@, @Ix@, and @Bounded@.

  Two forms of @deriving@ are possible. The first is used when a type only
  derives one class:

> data Priority = Low | Medium | High
>   deriving Show

  The second is used when multiple classes are derived:

> data Alarm = Soft | Loud | Deafening
>   deriving (Read, Show)

  It is a syntax error to specify @deriving@ for any other classes besides the
  seven given above.

  \sshd{Class Constraints}\label{class-constraints}

  Data types can be declared with class constraints on the type variables, but
  this practice is discouraged. It is better to hide the
  ``raw'' data constructors using the module system and instead export ``smart''
  constructors which apply appropriate constraints. In any case, the syntax used
  is:

> data (Num a) => SomeNumber a = Two a a
>   | Three a a a

  This declares a type @SomeNumber@ which has one type variable argument. Valid
  types are those in the @Num@ class.

\shd{Deriving}

  See the section on \hyperref[deriving]{@deriving@} under the @data@ keyword on
  page~\pageref{deriving}.

\shd{Do}\label{do}

  The @do@ keyword indicates that the code to follow will be in a \emph{monadic}
  context. Statements are separated by newlines, assignment is indicated by
  @<-@, and a @let@ form is introduced which does not require the @in@ keyword.

  \sshd{If and IO}\label{if-io}

  @if@ can be tricky when used with IO. Conceptually it is no different from an
  @if@ in any other context, but intuitively it is hard to develop. Consider the
  function @doesFileExist@ from @System.Directory@:

< doesFileExist :: FilePath -> IO Bool

  The @if@ statement has this ``signature'':

< if-then-else :: Bool -> a -> a -> a

  That is, it takes a @Bool@ value and evaluates to some other value based on
  the condition. From the type signatures it is clear that @doesFileExist@
  cannot be used directly by @if@:

< wrong fileName =
<   if doesFileExist fileName
<     then ...
<     else ...

  That is, @doesFileExist@ results in an @IO Bool@ value, while @if@ wants a
  @Bool@ value. Instead, the correct value must be ``extracted'' by running the
  IO action:

> right1 fileName = do
>   exists <- doesFileExist fileName
>   if exists
>     then return 1
>     else return 0

  Notice the use of @return@. Because @do@ puts us ``inside'' the @IO@ monad, we
  can't ``get out'' except through @return@. Note that we don't have to use @if@
  inline here---we can also use @let@ to evaluate the condition and get a value
  first:

> right2 fileName = do
>   exists <- doesFileExist fileName
>   let result =
>         if exists
>           then 1
>           else 0
>   return result

  Again, notice where @return@ is. We don't put it in the @let@ statement.
  Instead we use it once at the end of the function.

  \sshd{Multiple @do@'s}\label{multiple-dos}

  When using @do@ with @if@ or @case@, another @do@ is required if either branch
  has multiple statements. An example with @if@:

> countBytes1 f =
>   do
>     putStrLn "Enter a filename."
>     args <- getLine
>     if length args == 0
>       -- no 'do'.
>       then putStrLn "No filename given."
>       else
>         -- multiple statements require
>         -- a new 'do'.
>         do
>           f <- readFile args
>           putStrLn ("The file is " ++
>             show (length f)
>             ++ " bytes long.")

  And one with @case@:

> countBytes2 =
>   do
>     putStrLn "Enter a filename."
>     args <- getLine
>     case args of
>       [] -> putStrLn "No args given."
>       file -> do
>        f <- readFile file
>        putStrLn ("The file is " ++
>          show (length f)
>          ++ " bytes long.")

  An alternative syntax uses semi-colons and braces. A @do@ is still required,
  but indention is unnecessary. This code shows a @case@ example, but the
  principle applies to @if@ as well:

> countBytes3 =
>   do
>     putStrLn "Enter a filename."
>     args <- getLine
>     case args of
>       [] -> putStrLn "No args given."
>       file -> do { f <- readFile file;
>        putStrLn ("The file is " ++
>          show (length f)
>          ++ " bytes long."); }

\shd{Export}

  See the section on \hyperref[module]{@module@} on page~\pageref{module}.

\shd{If, Then, Else}\label{if}

  Remember, @if@ always ``returns'' a value. It is an expression, not just a
  control flow statement. This function tests if the string given starts with a
  lower case letter and, if so, converts it to upper case:

> -- Use pattern-matching to
> -- get first character
> sentenceCase (s:rest) =
>  if isLower s
>    then toUpper s : rest
>    else s : rest
> -- Anything else is empty string
> sentenceCase _ = []

\shd{Import}

  See the section on \hyperref[module]{@module@} on page~\pageref{module}.

\shd{In}

  See \hyperref[let]{@let@} on page~\pageref{let}.

\shd{Infix, infixl and infixr}

  See the section on \hyperref[operators]{operators} on
  page~\pageref{operators}.

\shd{Instance}

  See the section on \hyperref[class]{@class@} on page~\pageref{class}.

\shd{Let}\label{let}

  Local functions can be defined within a function using @let@. The @let@
  keyword must always be followed by @in@. The @in@ must appear in the same
  column as the @let@ keyword.  Functions defined have access to all other
  functions and variables within the same scope (including those defined by
  @let@). In this example, @mult@ multiplies its argument @n@ by @x@, which was
  passed to the original @multiples@. @mult@ is used by map to give the
  multiples of x up to 10:

> multiples x =
>   let mult n = n * x
>   in map mult [1..10]

  @let@ ``functions'' with no arguments are actually constants and,
  once evaluated, will not evaluate again for that invocation of the
  outer function. This is useful for
  capturing common portions of your function and re-using them. Here
  is a silly example which gives the sum of a list of numbers and
  their average.  The @numbers@ definition captures the list of
  numbers from @1@ to @m@, and will only be evaulated once per
  invocation of @listStats@; similarly, @total@ and @avg@ are only
  evaluated once per invocation:

> listStats m =
>   let numbers = [1 .. m]
>       total = sum numbers
>       avg = total / m
>   in "total: " ++ show total ++
>      ", avg: " ++ show avg

  \sshd{Deconstruction}\label{deconstruction}

  The left-hand side of a @let@ definition can also destructure its argument, in
  case sub-components are to be accessed. This definition would extract the
  first three characters from a string

> firstThree str =
>   let (a:b:c:_) = str
>   in "Initial three characters are: " ++
>       show a ++ ", " ++
>       show b ++ ", and " ++
>       show c

  Note that this is different than the following, which only works if the string
  has exactly three characters:

> onlyThree str =
>   let (a:b:c:[]) = str
>   in "The characters given are: " ++
>       show a ++ ", " ++
>       show b ++ ", and " ++
>       show c

\shd{Of}

  See the section on \hyperref[case]{@case@} on page~\pageref{case}.

\shd{Module}\label{module}

  A module is a compilation unit which exports functions, types, classes,
  instances, and other modules. A module can only be defined in one file, though
  its exports may come from multiple sources. To make a Haskell file a module,
  just add a module declaration at the top:

< module MyModule where

  Module names must start with a capital letter but otherwise can include
  periods, numbers and underscores. Periods are used to give sense of structure,
  and Haskell compilers will use them as indications of the directory a
  particular source file is, but otherwise they have no meaning.

  The Haskell community has standardized a set of top-level module names such as
  @Data@, @System@, @Network@, etc. Be sure to consult them for an appropriate
  place for your own module if you plan on releasing it to the public.

  \sshd{Imports}\label{imports}

  The Haskell standard libraries are divided into a number of modules. The
  functionality provided by those libraries is accessed by importing into your
  source file. To import everything exported by a library, just use the
  module name:

< import Text.Read

  Everything means \emph{everything}: functions, data types and constructors,
  class declarations, and even other modules imported and then exported by the
  that module. Importing selectively is accomplished by giving a list of names
  to import. For example, here we import some functions from @Text.Read@:

< import Text.Read (readParen, lex)

  Data types can be imported in a number of ways. We can just import the type and
  no constructors:

< import Text.Read (Lexeme)

  Of course, this prevents our module from pattern-matching on the values of
  type @Lexeme@. We can import one or more constructors explicitly:

< import Text.Read (Lexeme(Ident, Symbol))

  All constructors for a given type can also be imported:

< import Text.Read (Lexeme(..))

  We can also import types and classes defined in the module:

< import Text.Read (Read, ReadS)

  In the case of classes, we can import the functions defined for a class using
  syntax similar to importing constructors for data types:

< import Text.Read (Read(readsPrec
<                       , readList))

  Note that, unlike data types, all class functions are imported unless
  explicitly excluded. To \emph{only} import the class, we use this syntax:

< import Text.Read (Read())

  \sshd{Exclusions}\label{exclusions}

  If most, but not all, names are to be imported from a module, it would be
  tedious to list them all. For that reason, imports can also be specified via
  the @hiding@ keyword:

< import Data.Char hiding (isControl
<                         , isMark)

  Except for instance declarations, any type, function, constructor or class can
  be hidden.

  \sshd{Instance Declarations}\label{instance-declarations}

  It must be noted that @instance@ declarations \emph{cannot} be excluded from
  import: all @instance@ declarations in a module will be imported when the
  module is imported.

  \sshd{Qualified Imports}\label{qualified-imports}

  The names exported by a module (i.e., functions, types, operators, etc.) can
  have a prefix attached through qualified imports. This is particularly useful
  for modules which have a large number of functions having the same name as
  @Prelude@ functions. @Data.Set@ is a good example:

< import qualified Data.Set as Set

  This form requires any function, type, constructor or other name exported by
  @Data.Set@ to now be prefixed with the \emph{alias} (i.e., @Set@) given. Here
  is one way to remove all duplicates from a list:

> removeDups a =
>   Set.toList (Set.fromList a)

  A second form does not create an alias. Instead, the prefix becomes the module
  name. We can write a simple function to check if a string is all upper case:

< import qualified Char

> allUpper str =
>   all Char.isUpper str

  Except for the prefix specified, qualified imports support the same syntax as
  normal imports. The name imported can be limited in the same ways as described
  above.

  \sshd{Exports}\label{exports}

  If an export list is not provided, then all functions, types, constructors,
  etc. will be available to anyone importing the module. Note that any imported
  modules are \emph{not} exported in this case. Limiting the names exported is
  accomplished by adding a parenthesized list of names before the @where@
  keyword:

< module MyModule (MyType
<   , MyClass
<   , myFunc1
<   ...)
< where

  The same syntax as used for importing can be used here to specify which
  functions, types, constructors, and classes are exported, with a few
  differences. If a module imports another module, it can also export that
  module:

< module MyBigModule (module Data.Set
<   , module Data.Char)
< where
<
< import Data.Set
< import Data.Char

  A module can even re-export itself, which can be useful when all local
  definitions and a given imported module are to be exported. Below we export
  ourselves and @Data.Set@, but not @Data.Char@:

< module AnotherBigModule (module Data.Set
<   , module AnotherBigModule)
< where
<
< import Data.Set
< import Data.Char

\shd{Newtype}\label{newtype}

  While @data@ introduces new values and @type@ just creates synonyms, @newtype@
  falls somewhere between. The syntax for @newtype@ is quite restricted---only
  one constructor can be defined, and that constructor can only take one
  argument. Continuing the above example, we can define a @Phone@ type as
  follows:

> newtype Home = H String
> newtype Work = W String
> data Phone = Phone Home Work

  As opposed to @type@, the @H@ and @W@ ``values'' on @Phone@ are \emph{not}
  just @String@ values. The typechecker treats them as entirely new types. That
  means our @lowerName@ function from above would not compile. The following
  produces a type error:

< lPhone (Phone hm wk) =
<   Phone (lower hm) (lower wk)

  Instead, we must use pattern-matching to get to the ``values'' to which we
  apply @lower@:

> lPhone (Phone (H hm) (W wk)) =
>   Phone (H (lower hm)) (W (lower wk))

  The key observation is that this keyword does not introduce a new value;
  instead it introduces a new type. This gives us two very useful properties:

  \begin{compactitem}
  \item No runtime cost is associated with the new type, since it does not
  actually produce new values. In other words, newtypes are absolutely free!

  \item The type-checker is able to enforce that common types such as @Int@ or
  @String@ are used in restricted ways, specified by the programmer.
  \end{compactitem}

  Finally, it should be noted that any @deriving@ clause which can be attached
  to a @data@ declaration can also be used when declaring a @newtype@.

\shd{Return}

  See \hyperref[do]{@do@} on page~\pageref{do}.

\shd{Type}\label{type}

  This keyword defines a \emph{type synonym} (i.e., alias). This keyword does
  not define a new type, like @data@ or @newtype@. It is useful for documenting
  code but otherwise has no effect on the actual type of a given function or
  value. For example, a @Person@ data type could be defined as:

<  data Person = Person String String

  where the first constructor argument represents their first name and the
  second their last. However, the order and meaning of the two arguments is not
  very clear. A @type@ declaration can help:

> type FirstName = String
> type LastName = String
> data Person = Person FirstName LastName

  Because @type@ introduces a synonym, type checking is not affected in any way.
  The function @lower@, defined as:

> lower s = map toLower s

  which has the type

< lower :: String -> String

  can be used on values with the type @FirstName@ or @LastName@ just as easily:

> lName (Person f l) =
>   Person (lower f) (lower l)

\shd{Where}\label{where}

  Similar to @let@, @where@ defines local functions and constants. The scope of
  a @where@ definition is the current function. If a function is broken into
  multiple definitions through pattern-matching, then the scope of a particular
  @where@ clause only applies to that definition. For example, the function
  @result@ below has a different meaning depending on the arguments given to the
  function @strlen@:

> strlen [] = result
>   where result = "No string given!"
> strlen f = result ++ " characters long!"
>   where result = show (length f)

  \sshd{Where vs. Let}\label{where-vs-let}

  A @where@ clause can only be defined at the level of a function definition.
  Usually, that is identical to the scope of @let@ definition. The only
  difference is when guards are being used. The scope of the @where@ clause
  extends over all guards. In contrast, the scope of a @let@ expression is only
  the current function clause \emph{and} guard, if any.

\hd{Contributors}\label{contributors}

  My thanks to those who contributed patches and useful suggestions:
Dave Bayer, Evgenij Belikov, Paul Butler, Jonas Devlieghere, Elisa Firth, Marc Fontaine, Brian
Gianforcaro, Cale Gibbard, Andrew Harris, Stephen Hicks, Kurt
Hutchinson, Johan Kiviniemi, Patrik Jansson, Adrian Neumann, Barak
Pearlmutter, Lanny Ripple, Markus Roberts, Holger Siegel, Falko
Spiller, Emiel van de Laar, Adam Vogt, Leif Warner, and Jeff Zaroyko.

\hd{Version}\label{version}

  This is version 2.9. The source can be found at GitHub
  (\url{http://github.com/m4dc4p/cheatsheet}). The latest released
  version of the PDF can be downloaded from
  \url{http://cheatsheet.codeslower.com}.  Visit CodeSlower.com
  (\url{http://blog.codeslower.com/}) for other projects and writings.

\end{multicols}
\end{document}

% vim:set tw=80: