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Re^2: Generate the perl sequence 1, 11, 111, ....

by jeanluca (Deacon)
on Oct 10, 2008 at 07:54 UTC ( #716377=note: print w/ replies, xml ) Need Help??


in reply to Re: Generate the perl sequence 1, 11, 111, ....
in thread Generate the perl sequence 1, 11, 111, ....

I wanted to post something similar

@ones =map("1" x $_, 0..10) ;
What I don't understood is how it is possible that map can accept different syntax. For example, is it possible to write a perl subroutine that does the same thing, myPerlSub BLOCK LIST
Thnx
LuCa

UPDATE: thnx for the explanation!!


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Re^3: Generate the perl sequence 1, 11, 111, ....
by AnomalousMonk (Monsignor) on Oct 10, 2008 at 09:07 UTC
    ... is it possible to write a perl subroutine that does the same thing, myPerlSub BLOCK LIST
    Absolutely. See perlsub and in particular Prototypes.
    >perl -wMstrict -le "sub mysub (&@) { my $coderef = shift; map $coderef->($_), @_; } my @out = mysub { $_ += 33 } 0 .. 5; print qq{@out}; " 33 34 35 36 37 38
Re^3: Generate the perl sequence 1, 11, 111, ....
by quester (Vicar) on Oct 10, 2008 at 09:34 UTC
    The odd-looking syntax
    mysubroutine {block} @list;
    is mentioned in perlsub. Look in the section on prototypes where it describes the "&" prototype character, particularly the sentence "An & requires an anonymous subroutine, which, if passed as the first argument, does not require the sub keyword or a subsequent comma" and also the examples "try", "catch", and "mygrep".
     
    On the other hand, it looks to me that map is somewhat more magical than the examples in perlsub; if you call
    map (sub { "1" x $_ }, (1..10));
    it returns an array of coderefs (references to subroutines) which is not at all what is needed here.
     
    Also, not all builtin functions have regular enough syntax to be expressed with a prototype at all. map is one of them:
    perl -wle 'print prototype("CORE::map")'
    prints
    Use of uninitialized value in print at -e line 1.
      On the other hand, it looks to me that map is somewhat more magical than the examples in perlsub; if you call
      map (sub { "1" x $_ }, (1..10));
      it returns an array of coderefs (references to subroutines) which is not at all what is needed here.

      I personally believe that indeed map and its sibling grep are more magical: precisely because as I wrote in my reply to the OP they accept an expression to be evaluated for each of the other parameters. But I don't understand what you mean with "which is not at all what is needed here" because your example does exaclty what I mean: I do expect it to return a list (not an array!) of coderefs. Only, it probably does not do what you expect in that those coderefs are... all the same coderef:

      C:\temp>perl -E "say for map sub {1 x $_} => 1..3" CODE(0x182a944) CODE(0x182a944) CODE(0x182a944)

      This is because $_ is a package variable and those anonymous subs are not closures: when you will use one of them, its $_ will be that in scope at the moment, not the one passed to map() when it was created. To obtain that you have to close over a lexical:

      C:\temp>perl -E "say for map { my $x=$_; sub {1 x $x} } 1..3" CODE(0x23aa7c) CODE(0x182aef4) CODE(0x182ad74)

      Alternatively, and this is the interesting point to be noted here, Perl 5.10 and highter support lexical $_:

      C:\temp>perl -E "my $_; say for map sub {1 x $_} => 1..3" CODE(0x23a97c) CODE(0x182a9bc) CODE(0x183bbbc)
      --
      If you can't understand the incipit, then please check the IPB Campaign.
Re^3: Generate the perl sequence 1, 11, 111, ....
by blazar (Canon) on Oct 11, 2008 at 09:49 UTC
    What I don't understood is how it is possible that map can accept different syntax. For example, is it possible to write a perl subroutine that does the same thing, myPerlSub BLOCK LIST
    Thnx
    LuCa

    UPDATE: thnx for the explanation!!

    I personally believe that you shouldn't be too surprised. Quite a lot of builtin functions accept different kinds of arguments. Just see the first one that springs to mind, e.g. perldoc -f split:

    split /PATTERN/,EXPR,LIMIT split /PATTERN/,EXPR split /PATTERN/ split Splits the string EXPR into a list of strings and returns +that list. By default, empty leading fields are preserved, and +empty trailing ones are deleted. (If all fields are empty, they +are considered to be trailing.)

    Ok, the case of map is somewhat more surprising: actually, not only is it possible, as others explained to you, to implement the syntax above with Prototypes (which indeed are there much to the objective of letting you write subs that behave like builtins) but it is currently impossible to implement the alternate syntax of map() i.e. namely that with an expression since that would be more akin to to a macro, but there's not provision for such a beast under Perl 5. Of course you can write a sub that accepts a generic argument as the first one and when you use it, then the argument can be an expression, but that would be evaluated and passed to the sub as a scalar, not as an expression to be evaluated by your sub itself.

    HTH.

    --
    If you can't understand the incipit, then please check the IPB Campaign.

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