Comparing and Matching

NAME

"Perl 5 to 6" Lesson 09 - Comparing and Matching

SYNOPSIS

    "ab"    eq      "ab"    True
    "1.0"   eq      "1"     False
    "a"     ==      "b"     True
    "1"     ==      1.0     True
    1       ===     1       True
    [1, 2]  ===     [1, 2]  False
    $x = [1, 2];
    $x      ===     $x      True
    $x      eqv     $x      True
    [1, 2]  eqv     [1, 2]  True
    1.0     eqv     1       False

    'abc'   ~~      m/a/    True
    'abc'   ~~      Str     True
    'abc'   ~~      Int     False
    Str     ~~      Any     True
    Str     ~~      Num     False
    1       ~~      0..4    True
    -3      ~~      0..4    False

DESCRIPTION

Perl 6 still has string comparison operators (eq, lt, gt, le, ge, ne; cmp is now called leg) that evaluate their operands in string context. Similarly all the numeric operators from Perl 5 are still there.

Since objects are more than blessed references, a new way for comparing them is needed. === returns only true for identical values. For immutable types like numbers or Strings that is a normal equality tests, for other objects it only returns True if both variables refer to the same object (like comparing memory addresses in C++).

eqv tests if two things are equivalent, ie if they are of the same type and have the same value. Two identically constructed data structures are equivalent.

Smart matching

Perl 6 has a "compare everything" operator, called "smart match" operator, and spelled ~~.

For immutable types it is a simple equality comparison. A smart match against a type object checks for type conformance. A smart match against a regex matches the regex. Matching a scalar against a Range object checks if that scalar is included in the range.

There are other, more advanced forms of matching: for example you can check if an argument list (Capture) fits to the parameter list (Signature) of a subroutine, or apply file test operators (like -e in Perl 5).

What you should remember is that any "does $x fit to $y?"-Question will be formulated as a smart match in Perl 6.

SEE ALSO

http://perlcabal.org/syn/S03.html#Smart_matching