Provided by: libsyntax-keyword-match-perl_0.09-1_amd64 
NAME
"Syntax::Keyword::Match" - a "match/case" syntax for perl
SYNOPSIS
use v5.14;
use Syntax::Keyword::Match;
my $n = ...;
match($n : ==) {
case(1) { say "It's one" }
case(2) { say "It's two" }
case(3) { say "It's three" }
case(4), case(5)
{ say "It's four or five" }
default { say "It's something else" }
}
DESCRIPTION
This module provides a syntax plugin that implements a control-flow block called
"match/case", which executes at most one of a choice of different blocks depending on the
value of its controlling expression.
This is similar to C's "switch/case" syntax (copied into many other languages), or syntax
provided by Switch::Plain.
This is an initial, experimental implementation. Furthermore, it is built as a non-trivial
example use-case on top of XS::Parse::Keyword, which is also experimental. No API or
compatibility guarantees are made at this time.
Experimental Features
Some of the features of this module are currently marked as experimental (even within the
context that the module itself is experimental). They will provoke warnings in the
"experimental" category, unless silenced.
use Syntax::Keyword::Match qw( match :experimental(dispatch) );
use Syntax::Keyword::Match qw( match :experimental ); # all of the above
KEYWORDS
match
match( EXPR : OP ) {
...
}
A "match" statement provides the controlling expression, comparison operator, and sequence
of "case" statements for a match operation. The expression is evaluated to yield a scalar
value, which is then compared, using the comparison operator, against each of the "case"
labels in the order they are written, topmost first. If a match is found then the body of
the labelled block is executed. If no label matches but a "default" block is present, that
will be executed instead. After a single inner block has been executed, no further tests
are performed and execution continues from the statement following the "match" statement.
The braces following the "match" block must only contain "case" or "default" statements.
Arbitrary code is not supported here.
Even though a "match" statement is a full statement and not an expression, it can still
yield a value if it appears as the final statment in its containing "sub" or "do" block.
For example:
my $result = do {
match( $topic : == ) {
case(1) { ... }
}
};
Comparison Operators
The comparison operator must be either "eq" (to compare cases as strings) or "==" (to
compare them as numbers), or "=~" (to compare cases using regexps).
On Perl versions 5.32 onwards, the "isa" operator is also supported, allowing dispatch
based on what type of object the controlling expression gives.
match( $obj : isa ) {
case(A::Package) { ... }
case(Another::Package) { ... }
}
Remember that comparisons are made in the order they are written, from the top downwards.
Therefore, if you list a derived class as well as a base class, make sure to put the
derived class before the base class, or instances of that type will also match the base
class "case" block and the derived one will never match.
class TheBase {}
class Derived :isa(TheBase) {}
match( $obj : isa ) {
case(TheBase) { ... }
case(Derived) {
# This case will never match as the one above will always happen first
}
}
Since version 0.08 the operator syntax is parsed using XS::Parse::Infix, meaning that
custom infix operators can be recognised, even on versions of perl that do not support the
full "PL_infix_plugin" mechanism.
case
case(VAL) { STATEMENTS... }
case(VAL), case(VAL), ... { STATEMENTS... }
A "case" statement must only appear inside the braces of a "match". It provides a block of
code to run if the controlling expression's value matches the value given in the "case"
statement, according to the comparison operator.
Multiple "case" statements are permitted for a single block. A value matching any of them
will run the code inside the block.
If the value is a non-constant expression, such as a variable or function call, it will be
evaluated as part of performing the comparison every time the "match" statement is
executed. For best performance it is advised to extract values that won't need computing
again into a variable or "use constant" that can be calculated just once at program
startup; for example:
use constant CONDITION => a_function("with", "arguments");
match( $var : eq ) {
case(CONDITION) { ... }
...
}
The ":experimental(dispatch)" feature selects a more efficient handling of sequences of
multiple "case" blocks with constant expressions. This handling is implemented with a
custom operator that will entirely confuse modules like "B::Deparse" or optree inspectors
like coverage tools so is not selected by default, but can be enabled for extra
performance in critical sections.
default
A "default" statement must only appear inside the braces of a "match". If present, it must
be the final choice, and there must only be one of them. It provides a block of code to
run if the controlling expression's value did not match any of the given "case" labels.
COMPARISONS
As this syntax is fairly similar to a few other ideas, the following comparisons may be
useful.
Core perl's given/when syntax
Compared to core perl's "given/when" syntax (available with "use feature 'switch'"), this
syntax is initially visually very similar but actually behaves very differently. Core's
"given/when" uses the smartmatch ("~~") operator for its comparisons, which is complex,
subtle, and hard to use correctly - doubly-so when comparisons against values stored in
variables rather than literal constants are involved. It can be unpredictable whether
string or numerical comparison are being used, for example. By comparison, this module
requires the programmer to specify the comparison operator. The choice of string or
numerical comparison is given in the source code - there can be no ambiguity.
Additionally, the "isa" operator is also permitted, which has no equivalent ability in
smartmatch.
Also, the "given/when" syntax permits mixed code within a "given" block which is run
unconditionally, or at least, until the first successful "when" statement is encountered.
The syntax provided by this module requires that the only code inside a "match" block be a
sequence of "case" statements. No other code is permitted.
Switch::Plain
Like this module, Switch::Plain also provides a syntax where the programmer specifies
whether the comparison is made using stringy or numerical semantics. "Switch::Plain" also
permits additional conditions to be placed on "case" blocks, whereas this module does not.
Additionally, the "isa" operator is also permitted, which has no equivalent ability in
"Switch::Plain".
C's switch/case
The C programming language provides a similar sort of syntax, using keywords named
"switch" and "case". One key difference between that and the syntax provided for Perl by
this module is that in C the "case" labels really are just labels. The "switch" part of
the statement effectively acts as a sort of computed "goto". This often leads to bugs
caused by forgetting to put a "break" at the end of a sequence of statements before the
next "case" label; a situation called "fallthrough". Such a mistake is impossible with
this module, because every "case" is provided by a block. Once execution has finished with
the block, the entire "match" statement is finished. There is no possibility of accidental
fallthrough.
C's syntax only permits compiletime constants for "case" labels, whereas this module will
also allow the result of any runtime expression.
Code written in C will perform identically even if any of the "case" labels and associated
code are moved around into a different order. The syntax provided by this module
notionally performs all of its tests in the order they are written in, and any changes of
that order might cause a different result.
TODO
This is clearly an early experimental work. There are many features to add, and design
decisions to make. Rather than attempt to list them all here it would be best to check the
RT bug queue at
<https://rt.cpan.org/Dist/Display.html?Name=Syntax-Keyword-Match>
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>