On page 259 of HOP, MJD is talking of a linked list whose nodes are only constructed when the subroutine, tail is evaluated. The code for tail is similar to the following: 

sub tail {
        my $list = shift;
        if ( ref( $list->[1] eq 'CODE' ) { return $s->[1]->() );
        else { return $s->[1] }
}

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The code to construct the list, a list of the numbers from m to n, is: 

sub upto {
        my ($m, $n) = @_;
        return if $m > $n;
        node( $m, promise { upto($m+1, $n) } );
}

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where 

sub promise (&) { $_[0] }

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and: 

sub node {
        my ($head, $tail) = @_;
        return [$head, $tail];
}

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According to MJD, the tail() function sees the promise and invokes the anonymous promise function, which in turn invokes upto($m+1, $n). When I try this code out on Fedora 9 with perl-5.10.0, I get the error: 

'Can't call method "promise" on an undefined value.'

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It is apparently parsing the block argument to promise as an indirect method call on the non-existent object { upto($m+1, $n) } If I wrap the block argument to promise in parentheses, I get the errors: 

Odd number of elements in anonymous hash at stream.pl line 19.
Use of uninitialized value in anonymous hash ({}) at stream.pl line 19
+.

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If I then disambiguate the thing in braces as a block with { return upto($m+1, $n) }, undef is returned. And if I replace the braces in the original line with parentheses, like 

        node( $m, promise ( upto($m+1, $n) ) );

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the function is eagerly evaluated, not lazily. So, what do I tell MJD? I think this probably worked before. Is this a problem with newer perls?

In reply to lazy evaluation of sub arg by drbean

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