NAME
Carp::Ensure - Ensure a value is of the expected type
SYNOPSIS
use Carp::Ensure( qw( is_a ) );
ensure('string', "Some arbitrary string") if DEBUG;
ensure('@integer', 1, 2, 3) if DEBUG;
ensure('@\integer', \1, \2, \3) if DEBUG;
my %word2Int = ( one => 1, two => 2, three => 3 );
my @ints = values(%word2Int);
my @wordsInts = ( keys(%word2Int), @ints );
ensure('\@integer', \@ints) if DEBUG;
ensure('@word|integer', %word2Int) if DEBUG;
ensure('%word=>integer', %word2Int) if DEBUG;
die("Unexpected type")
unless is_a('@word|integer', @wordsInts);
die("Unexpected type")
unless is_a('@\@word|integer', \@wordsInts, [ "four", 4 ]);
# Receives a string, a `Mail::Internet' object, a reference to a hash mapping
# strings to integers
sub someSub($$%) {
ensure([ qw( string Mail::Internet HASH %string=>integer ) ], \@_) if DEBUG;
my( $string, $object, %hash ) = @_;
# ...
}
DESCRIPTION
Most of the time it's a nice feature, that Perl has no really strict type checking as in C++. However, sometimes you want to ensure, that you subs actually get the type of arguments they expect. Or they return what you expect.
That is where Carp::Ensure may be useful. You can check every value whether it has the type you expect. You may fine tune the type checking from very coarse checking like defined vs. undefined to very detailed checks which check even the keys and values of a hash. In most places you may give alternative types so for instance a parameter can easily be checked to be of a certain type or undefined.
There are checking routines for a few commonly used base types included and you may add your own checking routines so you can check for the types specific to your program.
The types are described by a simple grammar which picks up as much as possible you already know from the Perl type system.
FUNCTIONS
ensure
use Carp::Ensure;
ensure("some_type", $value) if DEBUG;
ensure("@value_type", @array) if DEBUG;
ensure("%key_type=>value_type", %hash) if DEBUG;
ensure([ qw( type1 type2 ... ) ], [ $value1, $value2, ... ]) if DEBUG;
ensure([ qw( type1 type2 ... ) ], \@_) if DEBUG;
Checks whether the types described in the first argument are
matched by the values given in the following arguments. If
the values match the type ensure returns an aribtrary value.
If a value doesn't match the specified type, ensure
Carp::confesses with an approriate error message and thus
stops the program.
If the first argument is a string, it describes the type of
the remaining arguments which may be arbitrary many
(including none). This is useful for list types (i.e. arrays
and hashes) and to check single values.
If the first argument is a reference to an array, the second
argument must be a reference to an array, too. In this
calling scheme the first array describes the types contained
in the second argument. It is particularly useful to check
the argument list of a sub.
Care is taken to not change the second argument in any way.
Note, that usually ot only makes sense when the last of the
described types checks for a list type. This is because in
Perl a list type sucks up all the remaining values.
See the section on "TYPE GRAMMAR" for how the types are
described.
The `if DEBUG' concept is taken from the Carp::Assert
manpage where it is explained in detail (particularly in the
section on "Debugging vs Production" in the Carp::Assert
manpage. Actually the DEBUG value is probably shared between
the Carp::Assert manpage and this module. So take care when
enabling it in one and disabling it in the other package
`use'. In short: If you say `use Carp::Ensure' you switch
DEBUG on and ensure works as expected. If you say `no
Carp::Ensure' then the whole call is compiled away from the
program and has no impact on efficiency.
is_a
# Both are possible
use Carp::Ensure( qw( :DEBUG is_a ) );
use Carp::Ensure( qw( :NDEBUG is_a ) );
$is_of_type = is_a("some_type", $value);
$is_of_type = is_a("@value_type", @array);
$is_of_type = is_a("%key_type=>value_type", %hash);
$is_of_type = is_a([ qw( type1 type2 ... ) ], [ $value1, $value2, ... ]);
$is_of_type = is_a([ qw( type1 type2 ... ) ], \@_);
This does the same as ensure, however, it only returns true
or false instead of Carp::confessing. You can use this to
check types of values without immediately stopping the
program on failure or to build your own testing subs like
this:
sub Carp::Ensure::isaword1empty { Carp::Ensure::is_a('word|empty', ${shift()}) }
If a false value is returned $@ is set to an error
message. Otherwise $@ is undefined.
TYPE GRAMMAR
You may create rather complex type descriptions from the following grammar.
Lexical rules
Since whitespace is not relevant in the grammar, it may occur anywhere outside of identifiers. Actually any whitespace is removed before parsing the type description starts.
Grammar rules
type :=
hash | array | alternative
hash :=
'`%'' alternative '`='>' alternative
array :=
'`@'' alternative
alternative :=
simple '`|'' alternative | simple
simple :=
reference | dynamic | special | scalar
reference :=
'`\'' type | class | object | '`HASH'' | '`ARRAY'' |
'`CODE'' | '`GLOB''
Note: Take care with the `\'. Even in a string using single
quotes a directly following backslash quotes a backslash!
Whitespace between subsequent backslashes simplifies things
greatly.
dynamic :=
user
special :=
'`undefined'' | '`defined'' | '`anything''
scalar :=
'`string'' | '`word'' | '`empty'' | '`integer'' | '`float''
| '`boolean'' | '`regex''
These common simple types are predefined.
class :=
'`^'' object
A value matching such a type is a name of a class (i.e. a
string) represented by the name matching the regular
expression object. This may mean, that the class is a
superclass of the class given by the value.
Thus the first parameter of a method which might be used
static as well as with an object has a type of
Some::Class|^Some::Class
object :=
/^[A-Z]\w(::\w+)$/
The value is a object (i.e. a blessed reference) of the
class represented by the name matching the regular
expression. This may mean, that the class is a superclass of
the object's class.
user :=
/^[a-z]\w$/*
This might be a string userType matching the regular
expression. For this a sub
`Carp::Ensure::is_a_'userType
must be defined. When checking a value for being a
userType, the sub is called with a single argument being a
reference(!) to the value it should check. This minimizes
copying. The sub must return false if the referenced value
is not of the desired type and a true value otherwise. See
`is_a' for an example.
Terminal symbols
The terminal symbols have the following meaning:
`HASH'
The value is a reference(!) to a hash with arbitrary keys
and values. Use this if you don't want to check the hash
content.
`ARRAY'
The value is a reference(!) to an array with arbitrary
content. Use this if you don't want to check the array
content.
`CODE'
The value is a reference to some code. This may be an
anonymous or a named sub.
`GLOB'
The value is a GLOB.
`undefined'
Only the undefined value is permitted. Often used as one
part of an alternative. Missing optional arguments of a sub
are undefined, also.
`defined'
The value only needs to be defined.
`anything'
Actually not a test since anything is permitted.
`string'
An arbitrary string.
`word'
A string matching `/w+/'.
`empty'
An empty string.
`integer'
An integer.
`float'
An floating point number.
`boolean'
A boolean. Actually every scalar is a boolean in Perl, so
this is more a description of how a certain value is used.
`regex'
A string which compiles cleanly as a regular expression. The
`regex' is applied to an empty string so any parentheses in
the `regex' will probably don't result in anything useful.
Note, that nothing prevents the `regex' from executing
arbitrary code if you manage to include this somehow. The
results are completly undefined.
Precedence
The precedence of the operators is as indicated by the grammar. Because most operators are prefix operators there is not much room for ambiguity anyway. However, the grammar for alternatives opens some traps. In particular the current grammar means, that it is not possible to have
If you want to describe arrays consisting of exactly one or
another type use an additional reference for your value and
try `\@type1|\@type2'.
However, `%typeK='>`\@typeV1|\@typeV2' is possible and
describes a hash mapping `typeK' values to references to
arrays consisting of either `typeV1' or `typeV2' elements.
Even worse `\%typeK1|typeK2='>`typeV' can't be parsed at all
because the alternative is evaluated before the hash
designator.
Note, that you can always define your own test functions which may break down complex types to simple names. With the `is_a' function this is usually done with a few key strokes.
TODO
If parentheses, brackets and braces would be added to the grammar, the following changed productions would be probably
simple :=
'`('' alternative '`)' | reference | ...
reference :=
'`\'' simple | '`['' alternative '`]'' | '`{''
alternative '`='>' alternative '`}'' | class | ...
dynamic :=
user | '`/'' match '`/'' | number '`..'' number
match :=
a valid Perl regex
number :=
/^[-+]?\d+(\.\d)([Ee][-+]\d+)?$/*
so you can define an anonymous type for a string matching a
regex or for a number being inside a range. But given the
rich structure of Perl regexes at least the match would
require a real parser.
SIMILAR MODULES
There is the the Usage manpage package which has a similar functionality. However, it dates 1996 and seems not be maintained since then. Unfortunately it is not as flexible as this module and is still a bit buggy.
AUTHOR
Stefan Merten <smerten@oekonux.de>
The idea for the code implementing the DEBUG feature was taken from the Carp::Assert manpage by Michael G. Schwern <schwern@pobox.com>.
SEE ALSO
the Carp manpage
the Carp::Assert manpage
LICENSE
This program is licensed under the terms of the GPL. See
http://www.gnu.org/licenses/gpl.txt
AVAILABILTY
See