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\begin{center}
{\Huge\bf GOFER}
\vspace{5mm}

{\Large\bf Gofer 2.21 release notes}
\vspace{5cm}

{\Large Mark P.~Jones}
\vspace{10mm}

{\Large November 1991}
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This document is intended as a supplement to the user manual ``An
introduction to Gofer'' supplied with the previous public release of
Gofer, version 2.20.1.  It provides brief descriptions of the changes
and new features incorporated in version 2.21.

If you would like to be informed when bug-fixes or further versions
become available, please contact me at \verb"jones-mark@cs.yale.edu" (if you
have not already done so) and I will add your name to the list.

Please contact me if you have any questions about the Gofer system, or
if you need some advice or help to complete a port of Gofer to a new
platform.

In addition to PC and Sun workstations, I have now had reports that
Gofer has been successfully compiled and used on a number of other
machines including Apollo, DecStation, Mips, MicroVax and Acorn ARM
machines, with little or no changes to the original source.

This \LaTeX\ version of the release notes was prepared
by Jeroen Fokker, \verb"jeroen@cs.ruu.nl".

\subsubsection*{Acknowledgements}
Many of the features described in this document were motivated by
comments and suggestions from users of the previously released version
of Gofer.  My thanks in particular to Julian Seward, but also to Brent
Benson, Stuart Clayman, Andy Gill, Peter Hancock, Ian Holyer, Hiroyuki
Matsuda, Aiden McCaughey, Tobias Nipkow, Will Partain, Ian Poole,
Bernard Sufrin and Phil Wadler.

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\section{Minor enhancements}

A number of small enhancements have been made to make the source code
for Gofer a little more flexible.  In particular, this includes:
\begin{itemize}
\item  Gofer can now be compiled using the Gnu C compiler gcc, for those
     who prefer this to the standard cc compiler provided on their
     machine.
\item  Default table sizes for the Unix version have been expanded which
     makes it possible to support larger programs in Gofer (a program
     of over 5000 lines has already been tested with this release).
\item  The Makefile has been made less SunOS specific and should be
     usable on a wider range of machines without modification.
\end{itemize}





\section{User interface extensions}

The user interface of the previous release has been extended to support
a range of new features, intended to make the Gofer environment more
convenient for program development.  Further details are given in the
following sections.

\subsection{Command line options}
Although the previous version of Gofer accepted some command line
options, these were not documented.  Those who discovered about the
Gofer command line options in the previous release by reading the
source code should note that a different syntax is now used which is
not compatible with the older system.

Options may be set when loading Gofer (on the UNIX/DOS command line)
or within the interpreter itself using the :set command.  Using this
command on its own with no arguments prints a menu of all of the
available options and displays the current settings:
\begin{verbatim}
    ? :set
    Groups of options begin with +/- to turn options on/off resp.
 
    TOGGLES:
    s    Print no. reductions/cells after eval
    t    Print type after evaluation
    d    Show dictionary values in output exprs
    f    Terminate evaluation on first error
    g    Print no. cells recovered after gc
    c    Test conformality for pattern bindings
    l    Treat input files as literate scripts
    e    Warn about errors in literate scripts
    i    Apply fromInteger to integer literals
    o    Optimise use of (&&) and (||)
    u    Catch ambiguously typed top-level vars
    a    Use any evidence, not nec. best
    E    Fail silently if evidence not found

    OTHER OPTIONS: (leading + or - makes no difference)
    hnum Set heap size (cannot be changed within Gofer)
    pstr Set prompt string to str
    xnum Set maximum depth for evidence search

    Current settings: +sdcoaE -tfgleiu -h100000 -p? -x8
    ?
\end{verbatim}
Most options are toggles meaning that they can either be switched on
(by preceding the option with a `\verb"+"' character) or off 
(by using a `\verb"-"'
character).  Several options may be grouped together so that:
\begin{verbatim}
  :set +std -le     is equivalent to       :set +s +t +d -l -e
\end{verbatim}
In order to distinguish command line options from filenames, a leading
`\verb"+"' or `\verb"-"' must also be used with the `\verb"h"', `\verb"p"' 
and `\verb"x"' options, although
the choice in each case is not significant.

Options may also be used in \verb":a" and \verb":l" commands, 
and within project files
(see section 2.2), although it should be noted that they will be acted
upon as soon as they are encountered and will not be reapplied when
reloading files.

Most of the options listed above are described in more detail in the
following sections.


\subsubsection{Set Gofer prompt}
The standard Gofer prompt `\verb"? "' may be changed using a command line
option of the form \verb"-p"{\em str} where for any string {\em str}.  
The new prompt is
formed from the given string, followed by a single space:
\begin{verbatim}
    ? :set -pGofer>
    Gofer> :set -p?
    ?
\end{verbatim}

\subsubsection{Print statistics}
In normal operation, Gofer displays the number of reductions and cells
used by a particular calculation when the result has been evaluated or
if the calculation is interrupted:
\begin{verbatim}
    ? map (\x -> x*x) [1..10]
    [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
    (112 reductions, 204 cells)
    ? [1..]
    [1, 2, 3, 4, ^C{Interrupted!}

    (18 reductions, 54 cells)
    ?
\end{verbatim}
Printing of these statistics can be suppressed using the \verb"-s" option
(and subsequently restored using \verb"+s"):
\begin{verbatim}
    ? :set -s
    ? map (\x -> x*x) [1..10]
    [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
    ? [1..]
    [1, 2, 3, 4, ^C{Interrupted!}

    ? :set +s
    ? 2 + 4
    6
    (2 reductions, 6 cells)
    ?
\end{verbatim}

\subsubsection{Print type}
Before evaluating an expression entered into the interpreter, the Gofer
type checker is used to determine the type of the resulting value.
This is used to detect errors in the original input expression, avoid
the use of runtime type checks and determine how the value should be
output.  The actual type of the term is not usually displayed unless a
type error is detected.  This behaviour can be changed using the \verb"+t"
option which displays the type of each value as soon as evaluation is
complete.
\begin{verbatim}
    ? :set +t
    ? map (\x -> x*x) [1..10]
    [1, 4, 9, 16, 25, 36, 49, 64, 81, 100] :: [Int]
    (108 reductions, 204 cells)

    ? map concat
    map concat :: [[[a]]] -> [[a]]
    (2 reductions, 17 cells)

    ? fix where fix f = f (fix f)
    v112 :: (a -> a) -> a
    (1 reduction, 7 cells)
    ?
\end{verbatim}
Note that values of type \verb"String" and \verb"Dialogue" 
(or equivalent forms) are
displayed in a different manner to other values, and no type information
is printed after such values to avoid any possibility of confusion:
\begin{verbatim}
    ? map                              -- the map function
    map :: (a -> b) -> [a] -> [b]
    (1 reduction, 6 cells)

    ? "map"                            -- a string expression
    map
    (0 reductions, 4 cells)

    ? print "map"                      -- a dialogue
    "map"
    (18 reductions, 44 cells)
    ?
\end{verbatim}

\subsubsection{Show dictionaries}
The implementation of overloading in Gofer uses a translation of each
expression entered into the interpreter to a new expression involving
dictionary variables and constants.  These additional parameters are
usually included in expressions displayed by the interpreter and are
often useful for understanding and resolving overloading problems:
\begin{verbatim}
    ? \x -> x + x
    ERROR: Unresolved overloading
    *** type        : Num a => a -> a
    *** translation : \d125 x -> (+) d125 x x

    ? :t map (1+) [1..10]
    map ((+) {dict} 1) (enumFromTo {dict} 1 10) :: [Int]
    ?
\end{verbatim}
If necessary (perhaps to make the output of Gofer easier for a beginner
to understand), the printing of dictionary parameters may be suppressed
using the \verb"-d" option:
\begin{verbatim}
    ? :set -d
    ? \x -> x + x
    ERROR: Unresolved overloading
    *** type        : Num a => a -> a
    *** translation : \x -> x + x

    ? :t map (1+) [1..10]
    map (1 +) (enumFromTo 1 10) :: [Int]
    ?
\end{verbatim}
The original behaviour can be obtained using \verb":set +d" within the
interpreter.


\subsubsection{Terminate on error}
When an irreducible subexpression is encountered during the evaluation
of a particular expression, the irreducible redex is printed with
surrounding braces and the Gofer interpreter attempts to continue the
evaluation with other parts of the original expression:
\begin{verbatim}
    ? take (1/0) [1..]          -- value is bottom
    {primDivInt 1 0}
    (4 reductions, 33 cells)
    ? [1/0]                     -- value is [bottom]
    [{primDivInt 1 0}]
    (5 reductions, 34 cells)
    ? [1/0, 2]                  -- value is [bottom, 2]
    [{primDivInt 1 0}, 2]
    (7 reductions, 43 cells)
    ?
\end{verbatim}
Notice that, reading an expression enclosed in \{braces\} as bottom, each
of the values printed by Gofer gives the correct value.  Of course, it
is not possible to arrange for anything to be printed when a value of
bottom is generated by a nonterminating computation:
\begin{verbatim}
    ? last [1..]
    ^C{Interrupted!}            -- nothing printed until interrupted

    (10470 reductions, 15712 cells)
    ?
\end{verbatim}
An alternative behaviour is provided by the \verb"+f" option, which causes the
evaluation of an expression to be abandoned completely if an error
occurs:
\begin{verbatim}
    ? :set +f
    ? take (1/0) [1..]

    Aborting evaluation: {primDivInt 1 0}
    (3 reductions, 55 cells)

    ? [1/0]

    Aborting evaluation: {primDivInt 1 0}
    (3 reductions, 54 cells)
 
    ? [1/0,2]

    Aborting evaluation: {primDivInt 1 0}
    (3 reductions, 56 cells)

    ?
\end{verbatim}
Note that we are no longer able to distinguish between the values
produced by these three terms from the output produced by Gofer -- the
only differences are in the number of reductions and cells used which
tells us nothing about the values of the terms.  Note that the basic
method of evaluation in Gofer is unchanged -- the \verb"+f" option simply
modifies the printing mechanism (i.e the means by which values are
displayed) to be more strict (in the technical sense of the word).

Although the use of the \verb"+f" option makes the Gofer printing mechanism
less accurate, it is sometimes useful to use this option during program
development so that an error can be detected as soon as it occurs.  The
original behaviour can of course be restored at any time using the \verb"-f"
option.


\subsubsection{Heap size}
The \verb"-h"{\em number} option can be used to set the heap size 
(i.e. total number
of cells available at any one time), but cannot be used once the
interpreter has been loaded.  For example, starting the interpreter
with the command:
\begin{verbatim}
    gofer -h20000
\end{verbatim}
will typically start the Gofer interpreter with a heap of 20000 cells.
Note that the heap is used to hold an intermediate (parsed) form of an
input file while it is being read, type checked and compiled.  It
follows that, the larger the input file, the larger the heap required
to enable that file to be loaded into Gofer.  In practice, most large
programs are written (and loaded) as a number of separate files (see
section 2.2) which means that this does not usually cause problems.


\subsubsection{Garbage collector notification}
It is sometimes helpful to be able to tell when the garbage collector
is being used, in order to monitor the amount of time involved and the
number of cells recovered with each garbage collection.  If the \verb"+g"
command line option is given (for example, using the command \verb":set +g")
then the garbage collector prints a message of the form \verb"{{Gc:num}}" each
time that the garbage collector is invoked.  The number after the colon
indicates the total number of cells that have been recovered.

The garbage collector messages are actually printed in three sections,
which indicate which stage the garbage collector has reached (this is
only noticeable on slower machines of course!):
\begin{quote}
\begin{tabular}{ccccccc}
            & \verb"{{Gc" &&   \verb":" &&             \verb"number}}" \\
   garbage    &&   marking cells  &&    preparing     &&      garbage    \\
   collection &&     which are    &&   unused cells   &&      collection \\
   begins     &&    still in use  &&    for reuse     &&      completed
\end{tabular}
\end{quote}
Garbage collector messages may be printed at almost any stage in a
computation (or indeed whilst loading, type checking or compiling a
file of definitions).  For this reason, it is often better to turn
the garbage collector messages off (using \verb":set -g" for example) when
they are not required.


\subsubsection{Conformality testing}
As described briefly in section 9.11 of the documentation for Gofer
version 2.20, pattern bindings of the form \I{pat}\verb"="\I{expr}
are implemented
using a `conformality check' to ensure that the value of expr does
indeed match the pattern pat.  For example, the pattern binding:
\begin{verbatim}
    (x:xs) = [1..]
\end{verbatim}
is actually implemented as if it had been defined by:
\begin{verbatim}
    (x:xs) = conformality [1..]
             where conformality v@(_:_) = v
\end{verbatim}
which is in turn treated as a group of bindings:
\begin{verbatim}
    xxs    = conformality [1..] where conformality v@(_:_) = v
    x      = head xxs
    xs     = tail xxs
\end{verbatim}
The variables conformality and \verb"xxs" used here are given as examples
only -- in practice, Gofer maintains a supply of variable names and
selects new names from this supply to avoid clashes with variables
which are already in use.

The conformality check does not cause any problems in the example
above because the list \verb"[1..]" is always guaranteed to match the
pattern \verb"(x:xs)" (i.e.\ a non-empty list).  We can however see the
conformality check in action is we try examples in which the pattern
does not match:
\begin{verbatim}
    ? x where (x:xs) = []
    {v114 []}
    (3 reductions, 25 cells)

    ? xs where (0:xs) = [1..]
    {v114 [1] ++ iterate (primPlusInt 1) (primPlusInt 1 1)}
    (13 reductions, 94 cells)
    ?
\end{verbatim}
The variable \verb"v114" in each of these examples is the variable name
representing the conformality check.  As the second example shows, the
value of the expression on the right hand side of the pattern binding
is evaluated as much as possible to determine whether the pattern
fits.

This example also demonstrates a small problem with the printer
in that, when the first element of the list is encountered, it is
unable to detect that the tail of the list has not yet been evaluated.
Consequently, the expression:
\begin{verbatim}
    [1] ++ iterate (primPlusInt 1) (primPlusInt 1 1)
\end{verbatim}
is not enclosed in parentheses as it should be.  This is a little
annoying, but not important because the expression only appears in an
error message.  The problem cannot in general be solved unless we avoid
the use of the [...] notation for enumerating the elements of a list.

The conformality check must be used for compatibility with Haskell.
However, it is sometimes useful to be able to suppress the conformality
check using the \verb"-c" option (for example, to use some programs written
for a language without conformality checks within Gofer):
\begin{verbatim}
    ? :set -c
    ? x where (x:xs) = []
    {_SEL (:) [] 1}
    (5 reductions, 36 cells)
    ? xs where (0:xs) = [1..]
    [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14^C{Interrupted!}

    (55 reductions, 146 cells)
    ?
\end{verbatim}
In the first example, the expression \verb"_SEL (:) [] 1" indicates that the
first component of an object constructed using the (:) operator is to
be extracted from the object \verb"[]".  Clearly this is impossible, and hence
the expression \verb"_SEL (:) [] 1" is irreducible.  In the second case, the
value of xs is equivalent to \verb"_SEL (:) (1:[2..]) 2" which reduces to
the infinite list \verb"[2..]" as shown, 
despite the fact that \verb"[1..]" does not
match the pattern \verb"(0:xs)".

The \verb"_SEL" function is used internally by Gofer and cannot be
entered directly into the the interpreter.  One particular reason for
this is that it is not in general possible to assign a sensible type
to \verb"_SEL".  
Constructor functions appearing as the first argument to \verb"_SEL"
are printed in the normal manner.  There is no standard syntax for
writing tuple constructors in Gofer or Haskell which are therefore
printed in the form \verb"(,,,)" for example, where the number of commas
indicates the number of components in the tuple.  In the following
example the constructor \verb"(,)" denotes the pairing constructor.
\begin{verbatim}
    ? f a b where f (n+1) y = n+y;  (a,b) = (0,1)
    {v113 0 (_SEL (,) (0,1) 2)}
    (10 reductions, 63 cells)
    ?
\end{verbatim}
The same notation is sometimes used in the messages produced when type
errors are detected:
\begin{verbatim}
    ? (1,2) 3
    ERROR: Type error in application
    *** expression     : (1,2) 3
    *** term           : (,)
    *** type           : a -> b -> (a,b)
    *** does not match : c -> d -> e -> f

    ?
\end{verbatim}
This syntax for tuple constructor functions cannot be used in
expressions entered directly into Gofer.  It may however be a nice
extension to consider for future versions, allowing definitions such
as \verb"zip = zipWith (,)"  and  \verb"distl x = map (x,)."


\subsubsection{Literate scripts}
In common with most programming languages, Gofer typically treats input
from a file as a list definitions in which program text is the norm,
and comments play a secondary role, introduced by the character
sequences `\verb"--"' and `\verb"{- ... -}"'.

An alternative approach, using an idea described by Knuth as `literate
programming', gives more emphasis to comments and documentation, with
additional characters needed to distinguish program text from comments.
Gofer supports a form of literate programming based on an idea due to
Richard Bird and originally implemented as part of the functional
programming language Orwell.  The same idea has subsequently been
adopted by several other functional language systems.

A literate script contains a sequence of lines.  Program lines are
distinguished from comments by a `\verb">"' character in the first column.
This makes it particularly easy to write a document which is both an
executable program script and at the same time, without need for any
preprocessing, suitable for use with document preparation software such
as LaTeX.  Indeed, this document is itself a literate script containing
the following definition of the squaring function.
\begin{verbatim}
> sqr x = x * x
\end{verbatim}
The \verb"+l" option sets Gofer to treat each input file as a literate
script.  It should not be used on the command line unless the prelude
file has been edited to make a literate script.

The effect of using literate scripts can be thought of as applying a
preprocessor to each input file before it is loaded into Gofer.  This
program has a particularly simple definition in Gofer:
\begin{verbatim}
   illiterate   :: String -> String
   illiterate cs = unlines [ xs | ('>':xs) <- lines cs ]
\end{verbatim}
The system of literate scripts used in Orwell is actually a little more
complicated than this and requires that the programmer adopt two simple
conventions in an attempt to try to catch simple errors in literate
scripts:
\BI
\IT Every input file must contain at least one line whose first
     character is `\verb">"'.  This means that programs containing no
     definitions (because the programmer has forgotten to use the `\verb">"'
     character to mark definitions) are not being accepted.

\IT Lines containing definitions must be separated from comment lines
     by one or more blank lines (i.e. lines containing only space and
     tab characters).  This is useful for catching programs where the
     leading `\verb">"' character has been omitted from one or more lines in
     the definition of a function.  For example:
\begin{verbatim}
     >   map f []     = []
         map f (x:xs) = f x : map f xs
\end{verbatim}
     would result in an error if the `\verb">"' character appeared in the first
     column of the first line.
\EI
Gofer will report on errors of this kind if the \verb"+l" option is combined
with the \verb"+e" option (for example as \verb"+le").


\subsubsection{Optimise {\tt (\&\&)} and {\tt (||)}}
The operator symbols \verb"(&&)" and \verb"(||)" 
are usually used to represent the
boolean connectives conjunction (and) and disjunction (or).  By
default, Gofer uses the following equations to produce better code for
expressions involving these operators:
\begin{verbatim}
         x && y  =  if  x  then  y     else  False
         x || y  =  if  x  then  True  else  y
\end{verbatim}
This optimisation is only valid if the operator symbols \verb"(&&)" 
and \verb"(||)"
are indeed bound to the appropriate values at the top level (the
standard full definitions are required in order to support partial
applications involving these operators).  Although this optimisation is
in general valid (because the appropriated definitions are included in
the standard prelude), it may be necessary in certain cases (for
example, when working with a non-standard prelude) to suppress the
optimisation using the \verb"-o" option.



\subsection{Project Files}
Project files provide a simple way to use programs which has been
spread across a number of source files.  Larger programs are often
written in this way, to separate the different components of the
program into smaller pieces which can be developed and tested
independently of other components.

A project file is a simple text file containing a list of program
filenames.  The project file may also contain comments using either of
the Gofer conventions for comments.  As a simple example, a simple
project file, in a file named \verb"miniProlog", suitable for the
stack-based version of the mini Prolog interpreter included as a
demonstration program with Gofer 2.21 is as follows:
\begin{verbatim}
    -- This is a project file suitable for loading the stack-based
    -- version of the mini Prolog interpreter into Gofer 2.21
    --
    -- Load into Gofer using the command:   :p miniProlog
    --        or from command line using:   gofer + miniProlog

    Parse       -- general purpose parsing library
    Interact    -- general purpose library for interactive programs
    PrologData  -- definition of main data structures
    Subst       -- substitutions and unification
    StackEngine -- inference engine
    Main        -- top level program
\end{verbatim}
As indicated in the comments at the top, there are two ways of using
this file with Gofer.  Within the interpreter we can use the command
\verb":p miniProlog".  Once this command has been entered, Gofer reads the
contents of the project file and then attempts to load each of the
files named.  In general, if a particular project file \verb"proj" contains
the options \verb"op1",\dots, \verb"opn" and the filenames 
\verb"f1",\dots, \verb"fm", then the
command \verb":p proj" is equivalent to the sequence of commands:
\begin{verbatim}
    :l                -- clear any previously loaded scripts
    :set op1 ... opn  -- set options
    :l f1 ... fm      -- load files
\end{verbatim}
The project file name may also be specified on the command line used to
start the interpreter by preceding the project file name with a single
`\verb"+"' character.  Note that there must be at least one space on each side
of the `\verb"+"'.  This may be combined with standard command line options,
but any additional filename arguments will be ignored.  Starting Gofer
with a command of the form "gofer + proj" is equivalent to starting
Gofer without the \verb"+ proj" arguments and then giving the command
\verb":p proj".

In addition, Gofer records the name of the project file and displays
this with the list of files loaded.  For example:
\begin{verbatim}
    Gofer session for: (project: miniProlog)
    /users/mpj/public/Gofer/prelude 
    Parse
    Interact
    PrologData
    Subst
    StackEngine
    Main
    ? 
\end{verbatim}
Once a project file has been selected, the command \verb":p" (without any
arguments) can be used to force Gofer to reread the project file and
load fresh copies of each of the files listed there.  There are two
places in which this is particularly useful:
\BI
\IT If the project file itself has been modified since the last time
     that it was read.
\IT  To force Gofer to reload all of the files in the project,
     regardless of the last time they were modified.
\EI
As usual, the \verb":r" command can be used to reload each of the files in the
current project without rereading the project file itself, and avoiding
the need to read certain files which have not been modified since the
previous time they were loaded.

The use of project files integrates smoothly with the other parts of
the Gofer environment.  As an example consider a project file proj
containing the four filenames \verb"f1", \verb"f2", \verb"f3" 
and \verb"f4", and suppose that the
file \verb"f3" contains an error of some kind.  This leads to the following
sequence of commands and results:
\begin{verbatim}
    :p proj       -- attempt to load project proj
                  -- reads filenames f1, f2, f3, f4 from proj
                  -- load definitions from f1
                  -- load definitions from f2
                  -- load definitions from f3 -- error occurs
                  -- error message printed
    :e            -- starts up editor at relevant line in f3
                  -- correct error
                  -- exit editor
                  -- load definitions from f3
                  -- load definitions from f4
\end{verbatim}
After just these two commands, the error in \verb"f3" has been corrected and
all of the files mentioned in proj have been loaded, ready for use.


\subsection{Other new features}

\subsubsection{{\tt :find} -- find definition}
The command \verb":f name" starts up an editor to allow you to inspect (and
possibly modify) the definition of a particular name from the files
currently loaded into Gofer.  If supported (using the \verb"EDITLINE"
variable), Gofer will attempt to initialise the editor so that the
cursor is initially positioned at the first line in the definition.
There are three possibilities:
\BI
\IT  If the name is defined by a function or variable binding then
     the cursor is positioned at the first line in the definition of
     the name (ignoring any type declaration, if present).

\IT  If the name is a constructor function, then the cursor is
     positioned at the first line in the definition of the
     corresponding data definition.

\IT  If the name represents an internal Gofer function, then the
     cursor will be positioned at the beginning of the standard
     prelude file.
\EI
Note that names of infix operators should be given without any
enclosing them in parentheses.  Thus \verb":f ++" starts an editor on the
standard prelude at the first line in the definition of \verb"(++)".


\subsubsection{{\tt :!} -- shell escape}
A command of the form \verb":! cmd" can be used to execute a specified
system command without leaving the Gofer interpreter.  For example,
\verb":! ls" (or \verb":! dir" on MS DOS machines) can be used to list the
contents of the current directory.

The command \verb":!" without any arguments starts a new shell:
\BI
\IT  On a unix machine, the \verb"SHELL" environment variable is used to
     determine which shell to use (the default is \verb"/bin/sh").

\IT  On an {\sc ms dos} machine, the \verb"COMSPEC" environment variable is used
     to determine which shell to use.  This is usually \verb"COMMAND.COM"
     and you may return to Gofer using the \verb"EXIT" command.
\EI
As usual, it is not possible to use a shell escape to change the
current working directory.  The \verb":cd" command described in the following
section can be used for this purpose.


\subsubsection{{\tt :cd} -- change directory}
The command \verb":cd dir" changes the current working directory to the path
given by \verb"dir".  This command is ignored if the pathname is omitted.


\subsubsection{{\tt :names} -- list names}
The \verb":n" command lists the names of variables and functions whose
definitions are currently loaded into the Gofer interpreter.  Using
this command without any arguments produces the list of all names
known to the system.  For example, with just the standard prelude
loaded we obtain:
\begin{verbatim}
    ? :n
    !! && * + ++ - . / /= : < <= == > >= AppendChan AppendFile Echo
    Failure False FormatError OtherError ReadChan ReadError ReadFile
    SearchError Str Success True WriteError WriteFile [] \\ ^ abort abs
    all and any appendChan appendFile asTypeOf break chr cjustify
    concat const copy curry cycle div done drop dropWhile echo elem
    enumFrom enumFromThen enumFromThenTo enumFromTo error even exit
    filter flip foldl foldl' foldl1 foldr foldr1 fromInteger fst fst3
    gcd head help id inRange index init insert interact isAlpha
    isAlphanum isAscii isControl isDigit isLower isPrint isSpace
    isUpper iterate last layn lcm length lines ljustify map max maximum
    merge min minimum mod negate not notElem nub null odd or ord
    otherwise primDivFloat primDivInt primEqFloat primEqInt
    primIntToFloat primLeFloat primLeInt primMinusFloat primMinusInt
    primMulFloat primMulInt primNegFloat primNegInt primPlusFloat
    primPlusInt primPrint print prints product products qsort range
    readChan readFile rem repeat reverse rjustify run scanl scanl'
    scanl1 scanr scanr1 show show' showChar showList showString shows
    showsPrec signum snd snd3 sort space span splitAt stdecho stderr
    stdin stdout strDispatch strict subtract succDispatch sum sums tail
    take takeUntil takeWhile thd3 toLower toUpper transpose uncurry
    undefined unlines until until' unwords words writeFile zip zip3
    zip4 zip5 zip6 zip7 zipWith zipWith3 zipWith4 zipWith5 zipWith6
    zipWith7 ||
    (201 names listed)
    ?
\end{verbatim}
Note that the names are listed in the standard alphabetical order.

The \verb":n" can also accept one or more pattern strings which limits the list
of names printed to those names matching one or more of the given
pattern strings:
\begin{verbatim}
    ? :n fold*
    foldl foldl' foldl1 foldr foldr1
    (5 names listed)
    ?
\end{verbatim}
Each pattern string consists of a string of characters and may use the
standard wildcard characters: `\verb"*"' (matches anything), `\verb"?"' 
(matches any
single character), `\verb"\c"' (matches exactly the character \verb"c") 
and ranges of
characters of the form `\verb"[a-zA-Z]"' etc.  For example:
\begin{verbatim}
    ? :n *ap* *[Cc]han \\\\ ?
    * + - . / : < > AppendChan ReadChan \\ ^ appendChan appendFile
    map readChan
    (16 names listed)
    ?
\end{verbatim}

\subsubsection{{\tt \$\$} -- recall last expression}
The previously entered expression can be recalled at any stage whilst
using the Gofer interpreter (even if the list of currently loaded files
has subsequently been changed) by using the operator symbol \verb"$$":
\begin{verbatim}
    ? 42
    42
    (1 reduction, 5 cells)
    ? [$$]
    [42]
    (3 reductions, 12 cells)
    ? [$$]
    [[42]]
    (5 reductions, 19 cells)
    ? ($$, length $$)
    ([[42]],1)
    (14 reductions, 43 cells)
    ?
\end{verbatim}
The \verb"$$" symbol is bound to a new value each time that an expression is
evaluated, or its type determined using the \verb":t" command:
\begin{verbatim}
    ? :t $$
    ([[42]],length [[42]]) :: ([[Int]],Int)
    ? :t map (1+) [1..10]
    map ((+) {dict} 1) (enumFromTo {dict} 1 10) :: [Int]
    ? $$
    [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
    (100 reductions, 189 cells)
    ?
\end{verbatim}
Note that \verb"$$" can also be used when the last expression entered used
a where clause (such expressions are simply translated into the
appropriate let expressions):
\begin{verbatim}
    ? fibs where fibs = 0:1:zipWith (+) fibs (tail fibs)
    [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55^C{Interrupted!}

    (41 reductions, 136 cells)
    ? :t $$
    let {...} in fibs :: [Int]
    ? take 5 $$
    [0, 1, 1, 2, 3]
    (24 reductions, 77 cells)
    ?
\end{verbatim}
Note that \verb"$$" expands to the unevaluated form of the expression, so that
a certain amount of computation may be repeated if \verb"$$" is used more than
once in a subsequent expression:
\begin{verbatim}
    ? sum [1..10]
    55
    (92 reductions, 130 cells)
    ? $$ + $$
    110
    (176 reductions, 254 cells)
    ? x + x where x = sum [1..10]
    110
    (89 reductions, 131 cells)
    ?
\end{verbatim}
Note that the value of \verb"$$" is updated after the expression has been parsed
but before it is type checked:
\begin{verbatim}
    ? 42
    42
    (1 reduction, 5 cells)
    ? 4)
    ERROR: Syntax error in input (unexpected `)')
    ? $$ 4
    ERROR: Type error in application
    *** expression     : 42 4
    *** term           : 42
    *** type           : Int
    *** does not match : a -> b

    ?
\end{verbatim}

\subsubsection{Command names}
Command names of the form \verb":X" (where \verb"X" 
represents an arbitrary capital
letter) are no longer supported.  Each command has a mnemonic full name
of the form \verb":name" which can be abbreviated to \verb":n" 
where `\verb"n"' is the first
letter of the full name.  The complete list of commands produced by the
\verb":?" command is as follows:
\begin{verbatim}
    ? :?
    LIST OF COMMANDS:  Any command may be abbreviated to :c where
    c is the first character in the full name.
 
    :set <options>      set command line options
    :set                help on command line options
    :?                  display this list of commands
    <expr>              evaluate expression
    :type <expr>        print type of expression
    :names [pat]        list names currently in scope
    :load <filenames>   load scripts from specified files
    :load               clear all files except prelude
    :also <filenames>   read additional script files
    :reload             repeat last load command
    :project <filename> use project file
    :edit <filename>    edit file
    :edit               edit last file
    :find <name>        edit file containing definition of name
    :! command          shell escape
    :cd dir             change directory
    :quit               exit Gofer interpreter
    ?
\end{verbatim}


\section{Language differences}

There are very few changes to the language supported by Gofer -- most
programs that ran correctly under the previous release should run
without any changes.  The features described in the following sections
are (for the most part) extensions to the previous version.

\subsection{$c*p$ and $p+k$ patterns}
Motivated by recent discussion on the Haskell mailing list, starting
with a posting from Tony Davie, Gofer now supports a more general form
of $n+k$ pattern, together with a new form of patter, $c*p$.  The syntax of
patterns is extended to include:
\begin{verbatim}
        pattern ::= .... | pattern + integer | integer * pattern
\end{verbatim}
Note that, in the previous version of Gofer, only variables were
permitted for the pattern $p$ in a $p+k$ pattern.  Certain restrictions are
placed on the constants $c$ and $k$ used in $c*p$ and $p+k$ patterns; Gofer
currently requires $c>1$ and $k>0$.

The semantics of these new patterns are described by the equations
(suggested by Kent Karlsson):
\begin{verbatim}
    case e0 of {p+k -> e; _ -> e'}
      = if e0 >= k then case e0-k of {p -> e; _ -> e'} else e'

    case e0 of {c*p -> e; _ -> e'}
      = if e0 >= 0 then  case e0 `divRem` c of {(p, 0) -> e; _ -> e'}
                   else  e'
\end{verbatim}
In Gofer, both forms of pattern match nonnegative integers only (there
is no possibility for overloading here as there is in Haskell).

These features are included in Gofer to enable experimentation with the
use of $c*p$ patterns.  They are not currently supported by Haskell, and
are subject to change as we gain more experience using them.  To
illustrate the potential uses for these extensions, here are two
examples provided by Tony Davie in his original message which can be
used in Gofer:
\begin{verbatim}
    x^^0       = 1                     -- fast exponentiation 
    x^^(2*n)   = xn*xn where xn = x^^n -- compare with definition
    x^^(2*n+1) = x * x^^(2*n)          -- of (^) in the prelude

    fib 1       = 1                    -- fast fibonnacci
    fib 2       = 1
    fib (2*n)   = (fib(n+1))^^2 - (fib(n-1))^^2
    fib (2*n+1) = (fib(n+1))^^2 + (fib n   )^^2
\end{verbatim}


\subsection{Errors during output}
If an error of the form \verb"error str" occurs during an output request in
a program using the facilities for I/O, the \verb"IOError" value passed to
the failure continuation is the \verb"(WriteError str)", rather than
\verb=(WriteError "{error str}")= as in the previous release.  This enables
further evaluation of the string \verb"str" (for example to produce a
compound error message by concatenating several strings together).

You are strongly advised to consider using the standard prelude
continuation \verb"exit" in your programs in place 
of the \verb"abort" predicate;
whereas \verb"abort" causes a program to terminate without any indication of
the problem, \verb"exit" attempts to print a suitable error message before
the program terminates.


\subsection{Type synonyms in predicates}
Type synonyms may now be used in predicates (The previous release
allowed only data constructors).  This means that programs such as the
\verb"cat" program described in section 14.2.6  of the Gofer
documentation can now be written as:
\begin{verbatim}
    class    Cat a        where cat  :: a -> Dialogue
    instance Cat String   where cat n = showFile n done
    instance Cat [String] where cat   = foldr showFile done

    showFile name cont = readFile name abort
                             (\s->appendChan stdout s abort cont)
\end{verbatim}
This uses the type synonym \verb"String" in place of the expanded 
form \verb"[Char]"
required by the original program.  Note that it is still not permitted
to define overlapping instances; an attempt to add an instance for 
\verb"Cat [Char]" to the above will not be accepted.


\subsection{Reporting on ambiguous types}
Class declarations whose member functions have ambiguous types are no
longer permitted.  For example, in the class declaration:
\begin{verbatim}
    class Box a where
        mem :: Int
\end{verbatim}
The type of the member function \verb"mem" 
is \verb"Box a => Int" which is ambiguous
and produces the error message:
\begin{verbatim}
    ERROR "examp" (line 3): Ambiguous type signature in class declaration
    *** ambiguous type : Box a => Int
    *** assigned to    : mem

    ?
\end{verbatim}
Similar error messages are also produced an explicit type signature
includes an ambiguous type.  For example:
\begin{verbatim}
    func   :: Eq a => Int -> Int
    func x  = 2*x+1
\end{verbatim}
Results in an error of the form:
\begin{verbatim}
    ERROR "examp" (line 12): Ambiguous type signature in type declaration
    *** ambiguous type : Eq a => Int -> Int
    *** assigned to    : func

    ?
\end{verbatim}
By default, no error is signalled if an ambiguous type is assigned to a
variable or function by the type checker.  This makes it possible to
write definitions such as:
\begin{verbatim}
    f y xs  =  if xs==[] then 0 else g y
    g y     =  f y []
\end{verbatim}
The types obtained for each of these terms is as follows:
\begin{verbatim}
    f :: Eq [a] => b -> [a] -> Int
    g :: Eq [a] => b -> Int
\end{verbatim}
Note that the second type is ambiguous.  Making the analogy between
these mutually recursive functions and a two state machine, we can
think of a direct call to \verb"f" as initialising the machine correctly so
that there is no problem when we enter \verb"g".  On the other hand, entering
the system at \verb"g" does not initialise the machine correctly, as signalled
by the ambiguity.

Using the \verb"+u" command line flag forces Gofer to generate an error when
an attempt to assign an ambiguous type to a top-level function occurs.
For the above example this gives:
\begin{verbatim}
    ERROR "examp" (line 20): Ambiguous type signature in inferred type
    *** ambiguous type : Eq [a] => b -> Int
    *** assigned to    : g
 
    ?
\end{verbatim}
The restriction to top-level functions means that \verb"f" can still be
implemented by writing:
\begin{verbatim}
    f :: Eq [a] => b -> [a] -> Int
    f  = f'  where  f' y xs = if xs==[] then 0 else g y
                    g' y    = f y []
\end{verbatim}
which prevents external access to \verb"g'" (preventing entry to the finite
machine described above in state \verb"g").  Note that the type signature in
this example is necessary to avoid the monomorphism restriction.


\section{Other matters}

\subsection{Contributions}
I would like to hear from anyone with interesting Gofer programs or
other useful items which might be included (with full credit to the
original author(s) of course!) in subsequent releases of Gofer.  There
is already one example of this in the new release; a small gnuemacs
mode for running the Gofer interpreter from within gnuemacs on Unix
machines, contributed by Stuart Clayman.  See the file \verb"gofer.el" for
more details.

\subsection{Future directions}
There will not be another release of Gofer for some time.  There are
however a number of areas which I would like to investigate at some
point as extensions to the Gofer system:
\BI
\IT  The ability to use Haskell style type classes.
\IT  Facilities for working with modules, based on the approach
     described in the Haskell report.  A particular problem here is
     in finding an elegant way to provide the full power of the
     module system from the interactive environment.
\IT  The ability to write stand alone applications programs using
     Gofer.
\IT  An improved user interface.  There are a number of grand ideas
     based on the use of windowing, mouse, pulldown-menus etc.  The
     current user interface is closer to this kind of approach than
     might at first be realised.  More interesting ideas include the
     design of class, data type and value browsers, along the lines
     of a Smalltalk system.
\EI
I would be interested to hear from anyone with comments or suggestions
on any of these (or other ideas).


\end{document}