/
apdx-helpers.lyx
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apdx-helpers.lyx
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#LyX 1.6.10 created this file. For more info see http://www.lyx.org/
\lyxformat 345
\begin_document
\begin_header
\textclass book
\use_default_options false
\language english
\inputencoding auto
\font_roman default
\font_sans default
\font_typewriter default
\font_default_family default
\font_sc false
\font_osf false
\font_sf_scale 100
\font_tt_scale 100
\graphics default
\paperfontsize default
\spacing single
\use_hyperref false
\papersize default
\use_geometry false
\use_amsmath 1
\use_esint 1
\cite_engine basic
\use_bibtopic false
\paperorientation portrait
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\defskip medskip
\quotes_language english
\papercolumns 1
\papersides 1
\paperpagestyle default
\tracking_changes false
\output_changes false
\author ""
\author ""
\end_header
\begin_body
\begin_layout Chapter
Lift Helpers
\begin_inset CommandInset label
LatexCommand label
name "cha:Lift-Helpers"
\end_inset
\end_layout
\begin_layout Section
Introduction
\begin_inset CommandInset label
LatexCommand label
name "sec:Introduction"
\end_inset
\end_layout
\begin_layout Standard
Lift provides a fairly useful collection of helper artifacts.
The helpers are essentially utility functions that minimize the need for
boilerplate code.
This appendix is intended to introduce some of the more common utility
classes and objects to you so that you’re familiar with them.
If you would like more details, you can look at the API documentation for
the
\family typewriter
net.liftweb.util package
\family default
.
\end_layout
\begin_layout Section
Box (or Scala's Option class on steroids)
\begin_inset CommandInset label
LatexCommand label
name "sec:Box-(or-Scala's"
\end_inset
\end_layout
\begin_layout Standard
\family typewriter
net.liftweb.util.Box
\family default
(or Scala’s
\family typewriter
scala.Option
\family default
class on steroids) is a utility class that mimics Scala’s
\family typewriter
Option
\family default
type (also heavily used inside Lift).
To understand some of the underlying concepts and assumptions, let’s take
a quick look at
\family typewriter
Option
\family default
class first.
The
\family typewriter
Option
\family default
class allows a type-safe way of dealing with a situation where you may
or may not have a result.
\family typewriter
Option
\family default
has two values, either
\family typewriter
Some(value)
\family default
, where
\family typewriter
value
\family default
is actually the value, and
\family typewriter
None
\family default
, which is used to represent nothing.
A typical example for
\family typewriter
Option
\family default
is outlined using Scala’s Map type.
Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Option-and-Map"
\end_inset
shows a definition of a
\family typewriter
Map
\family default
, a successful attempt to get the value of key
\family typewriter
a
\family default
, and an attempt to get the value of key
\family typewriter
i
\family default
.
Notice that when we retrieved the existing key-value pair for
\family typewriter
a
\family default
, the value returned was
\family typewriter
Some(A)
\family default
and when we asked for the value of key
\family typewriter
i
\family default
, we received None.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Option and Map example
\begin_inset CommandInset label
LatexCommand label
name "lst:Option-and-Map"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
scala> val cap = Map("a" -> "A", "b" -> "B")
\end_layout
\begin_layout Plain Layout
cap: scala.collection.immutable.Map[java.lang.String,java.lang.String] =
\end_layout
\begin_layout Plain Layout
Map(a -> A, b -> B)
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
scala> cap.get("a")
\end_layout
\begin_layout Plain Layout
res1: Option[java.lang.String] = Some(A)
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
scala> cap.get("i")
\end_layout
\begin_layout Plain Layout
res2: Option[java.lang.String] = None
\end_layout
\end_inset
\end_layout
\begin_layout Standard
Getting the value out of an
\family typewriter
Option
\family default
is usually handled via Scala’s matching mechanism or via the
\family typewriter
getOrElse
\family default
function, as shown in Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Fetch-value-from"
\end_inset
:
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Fetch value from an Option
\begin_inset CommandInset label
LatexCommand label
name "lst:Fetch-value-from"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
def prettyPrint(foo: Option[String]): String = foo match {
\end_layout
\begin_layout Plain Layout
case Some(x) => x
\end_layout
\begin_layout Plain Layout
case None => "Nothing found."
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
// Which would be used in conjunction with the previous code:
\end_layout
\begin_layout Plain Layout
scala> prettyPrint(cap.get("a"))
\end_layout
\begin_layout Plain Layout
res7: String = A
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
scala> prettyPrint(cap.get("i"))
\end_layout
\begin_layout Plain Layout
res8: String = Nothing found.
\end_layout
\end_inset
\end_layout
\begin_layout Standard
\family typewriter
Box
\family default
in Lift covers the same base functionality as
\family typewriter
Option
\family default
but expands the semantics for missing values.
If we have an
\family typewriter
Option
\family default
that is
\family typewriter
None
\family default
at some point, we can’t really tell why that
\family typewriter
Option
\family default
is
\family typewriter
None
\family default
, although in many situations, knowing why would be quite helpful.
With
\family typewriter
Box
\family default
, on the other hand, you have either have a
\family typewriter
Full
\family default
instance (corresponding to
\family typewriter
Some
\family default
with
\family typewriter
Option
\family default
) or an instance that subclasses
\family typewriter
EmptyBox
\family default
(corresponding to
\family typewriter
None
\family default
).
\family typewriter
EmptyBox
\family default
can either be an
\family typewriter
Empty
\family default
instance or a
\family typewriter
Failure
\family default
instance incorporating the cause for the failure.
So you can think of
\family typewriter
Box
\family default
as a container with three states: full, empty, or empty for a particular
reason.
The
\family typewriter
Failure
\family default
case class takes three arguments: a
\family typewriter
String
\family default
message to describe the failure, a
\family typewriter
Box[Throwable]
\family default
for an optional exception related to the failure, and a
\family typewriter
Box[Failure]
\family default
for chaining based on earlier
\family typewriter
Failure
\family default
s.
\end_layout
\begin_layout Standard
As an example of how we can use
\family typewriter
Box
\family default
instances in real code, consider the case where we have to do a bunch of
null checks, perform an operation, and then perform more null checks, other
operations, and so on.
Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Pseudocode-nested-operations"
\end_inset
shows an example of this sort of structure.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Pseudocode nested operations example
\begin_inset CommandInset label
LatexCommand label
name "lst:Pseudocode-nested-operations"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
x = getSomeValue();
\end_layout
\begin_layout Plain Layout
if (x != null) {
\end_layout
\begin_layout Plain Layout
y = getSomeOtherValue();
\end_layout
\begin_layout Plain Layout
if (y != null) {
\end_layout
\begin_layout Plain Layout
compute(x, y);
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
\end_layout
\end_inset
\end_layout
\begin_layout Standard
This is tedious and error-prone in practice.
Now let’s see if we can do better by combining Lift’s
\family typewriter
Box
\family default
with Scala’s for comprehensions as shown in Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Scala-nested-operations"
\end_inset
.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Box nested operations example
\begin_inset CommandInset label
LatexCommand label
name "lst:Scala-nested-operations"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
def getSomeValue(): Box[Int] = Full(12)
\end_layout
\begin_layout Plain Layout
def getSomeOtherValue(): Box[Int] = Full(2)
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
def compute(x: Int, y: Int) = x * y
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
val res = for ( x <- getSomeValue();
\end_layout
\begin_layout Plain Layout
y <- getSomeOtherValue() if x > 10) yield compute(x,
y)
\end_layout
\begin_layout Plain Layout
println(res)
\end_layout
\end_inset
\end_layout
\begin_layout Standard
In Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Scala-nested-operations"
\end_inset
, we have two values,
\family typewriter
x
\family default
and
\family typewriter
y
\family default
, and we want to do some computation with these values.
But we must ensure that computation is done on the correct data.
For instance, the computation cannot be done if
\family typewriter
getSomeValue
\family default
returns no value.
In this context, the two functions return a
\family typewriter
Box[Int]
\family default
.
The interesting part is that if either or both of the two functions return
an
\family typewriter
Empty
\family default
Box instead of
\family typewriter
Full
\family default
(
\family typewriter
Empty
\family default
impersonating the nonexistence of the value), the
\family typewriter
res
\family default
value will also be
\family typewriter
Empty
\family default
.
However, if both functions return a
\family typewriter
Full
\family default
(like in Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Scala-nested-operations"
\end_inset
), the computation is called.
In our example the two functions return
\family typewriter
Full(12)
\family default
and
\family typewriter
Full(2)
\family default
, so
\family typewriter
res
\family default
will be a
\family typewriter
Full(24)
\family default
.
\end_layout
\begin_layout Standard
But we have something else interesting here: the
\family typewriter
if x > 10
\family default
statement (this is called a “guard” in Scala).
If the call to
\family typewriter
getSomeValue
\family default
returns a value less than or equal to 10, the
\family typewriter
y
\family default
variable won’t be initialized, and the res value will be
\family typewriter
Empty
\family default
.
This is just a taste of some of the power of using
\family typewriter
Box
\family default
for comprehensions; for more details on for comprehensions, see
\emph on
The Scala Language Specification
\emph default
, section 6.19, or one of the many Scala books available.
\end_layout
\begin_layout Standard
Lift’s
\family typewriter
Box
\family default
extends
\family typewriter
Option
\family default
with a few ideas, mainly the fact that you can add a message about why
a
\family typewriter
Box
\family default
is
\family typewriter
Empty
\family default
.
\family typewriter
Empty
\family default
corresponds to
\family typewriter
Option
\family default
’s
\family typewriter
None
\family default
and
\family typewriter
Full
\family default
to
\family typewriter
Option
\family default
’s Some.
So you can pattern match against a
\family typewriter
Box
\family default
as shown in Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Box-example"
\end_inset
.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Box example
\begin_inset CommandInset label
LatexCommand label
name "lst:Box-example"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
a match {
\end_layout
\begin_layout Plain Layout
Full(author) => Text("I found the author " + author.niceName)
\end_layout
\begin_layout Plain Layout
Empty => Text("No author by that name.")
\end_layout
\begin_layout Plain Layout
// message may be something like "Database disconnected."
\end_layout
\begin_layout Plain Layout
Failure(message, _, _) => Text("Nothing found due to " + message)
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
def confirmDelete {
\end_layout
\begin_layout Plain Layout
(for (val id <- param("id"); // get the ID
\end_layout
\begin_layout Plain Layout
val user <- User.find(id)) // find the user
\end_layout
\begin_layout Plain Layout
yield {
\end_layout
\begin_layout Plain Layout
user.delete_!
\end_layout
\begin_layout Plain Layout
notice("User deleted")
\end_layout
\begin_layout Plain Layout
redirectTo("/simple/index.html")
\end_layout
\begin_layout Plain Layout
}) getOrElse {error("User not found"); redirectTo("/simple/index.html")}
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
\end_layout
\end_inset
\end_layout
\begin_layout Standard
In conjunction with Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Box-example"
\end_inset
, we can use other
\family typewriter
Box
\family default
functions, such as the
\family typewriter
openOr
\family default
function shown in Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:openOr-example"
\end_inset
.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
openOr example
\begin_inset CommandInset label
LatexCommand label
name "lst:openOr-example"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
lazy val UserBio = UserBio.find(By(UserBio.id, id)) openOr (new UserBio)
\end_layout
\begin_layout Plain Layout
def view (xhtml: NodeSeq): NodeSeq = passedAuthor.map({ author =>
\end_layout
\begin_layout Plain Layout
// do bind, etc here and return a NodeSeq
\end_layout
\begin_layout Plain Layout
}) openOr Text("Invalid author")
\end_layout
\end_inset
\end_layout
\begin_layout Standard
We won't be detailing all of the Box functions here, but a few words on
the most common function might be benficial.
\end_layout
\begin_layout Standard
\align center
\begin_inset Tabular
<lyxtabular version="3" rows="10" columns="3">
<features>
<column alignment="left" valignment="top" width="1in">
<column alignment="left" valignment="top" width="2.75in">
<column alignment="left" valignment="top" width="2in">
<row>
<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
\begin_inset Text
\begin_layout Plain Layout
Function name
\end_layout
\end_inset
</cell>
<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none">
\begin_inset Text