Functional Composition

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Re: Functional Composition by ikegami (Patriarch) on Dec 17, 2009 at 21:10 UTC
'\|' is left associative, and the left argument is evaluated first Left-associative means that `a \| b \| c` [download] is equivalent to `( a \| b ) \| c` [download] and not `a \| ( b \| c )` [download] It says nothing about the order in which a, b and c are evaluated in relation to each other. In fact, the operand evaluation order is undefined for most Perl operators including the "\|" operator. Perl is free to evaluate `a \| b \| c` [download] as follows: `push @stack, c; push @stack, b; push @stack, a; push @stack, pop(@stack) \| pop(@stack); push @stack, pop(@stack) \| pop(@stack); pop @stack;` [download] The left pipe is evaluated before the right pipe, so the operator is left-associative, but note how c is evaluated before b, which is evaluated before a. Your code is relying on undefined functionality. It's not guaranteed to work. You'd have to switch to `\|\|` if you wanted predictable results. `\|\|` is guaranteed to evaluate its LHS argument before its RHS argument due to its short-circuiting nature. That's the official line, anyway. In reality, `\|`'s LHS arg is always evaluated before its RHS, and I don't see that ever changing.	[reply] [d/l] [select]
Re^2: Functional Composition by billh (Pilgrim) on Dec 17, 2009 at 21:19 UTC
I stand corrected, I chose '\|' pretty arbitrarily to mimic unix pipes, and assumed that the left operand would always be evaluated first. I do know what "left associative" means though, there was an "and" in that sentance :-) Bill H `perl -e 'print sub { "Hello @{[shift->()]}!\n" }->(sub{"World"})'` [download]	[reply] [d/l]
Re^3: Functional Composition by ikegami (Patriarch) on Dec 17, 2009 at 21:22 UTC
there was an "and" in that sentance There was a possibility that you meant "and therefore". If not, then you can ignore the top half while others benefit from it.	[reply]
Re^4: Functional Composition by billh (Pilgrim) on Dec 17, 2009 at 22:02 UTC
Re^5: Functional Composition by ikegami (Patriarch) on Dec 18, 2009 at 01:09 UTC
Re: Functional Composition by BrowserUk (Patriarch) on Dec 17, 2009 at 21:04 UTC
The same as this, but very, very slowly! `c:\test>perl -E"say join'-',split'', reverse ucfirst 'hello';" o-l-l-e-H` [download] Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "I'd rather go naked than blow up my ass"	[reply] [d/l]
Re^2: Functional Composition by billh (Pilgrim) on Dec 17, 2009 at 21:38 UTC
You're right of course, but I chose perl builtins to keep the example short. If you consider that each of those 'fn's could be arbitrarily complex and require braces around their arguments that would become `perl -E"say(join('-', split('', reverse(ucfirst('hello')))))"` [download] Plus I have to admit I kind of screwed up, "functional composition" really means "take two functions and produce a third", like `compose(sub{ ucfirst $[0] }, sub { $_[0] . "\n" });` [download] would return a function that would ucfirst its arg and append a newline, whereas my code invokes the functions directly. Still, it looks pretty. Bill H `perl -e 'print sub { "Hello @{[shift->()]}!\n" }->(sub{"World"})'` [download]	[reply] [d/l] [select]