evan_tech -- ruby blocks

12:53 pm, 6 Jan 06

ruby blocks

A friend has been learning Ruby and asking questions. His first: why are procs and blocks different? (More simply: why aren't blocks special syntax for procs?)

All I could come up with is that "return" in a block returns from the lexically enclosing function, while "return" in a proc returns from the proc itself. (This is a useful distinction, though it leads to the confusing LocalJumpError exception when you call a block after control has passed away from its lexically enclosing function...)

Is that it?

vanbeast	Do you mean "why aren't blocks special syntax for procs?"? I always thought that they were. I wasn't aware there was a distinction. Is a proc any different than a block with an anonymous enclosing function?
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evan	Thanks, fixed. I don't know the answer to your latter question.
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vanbeast	From Eric's comment below, it seems like it may be the case, but there's also a good chance I'm missing some subtlety here. I'm really curious about this now...
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drbrain

Blocks are mostly a syntax construct. Blocks get some special handling when you call a method with them that a proc doesn't get. You can't yield to a proc unless you convert it to a block first:

$ irb
> def foo() yield 'hi'; end
> foo(proc { |x| p x })
ArgumentError: wrong number of arguments (1 for 0)
        from (irb):2:in `foo'
        from (irb):2
> foo { |x| p x }
"hi"
> foo(&proc { |x| p x })
"hi"

But they're really the same thing inside:

> def bar(&block) p block end
> bar { }
#<Proc:0x00000000@(irb):6>
> bar
nil

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mart

This is interesting because I was thinking earlier about this very problem for the hypothetical “perfect programming language” I've been working on. In the end though, I distracted myself with the realisation that my syntax doesn't permit anonymous functions to look like blocks because the parameters need to be named somehow. Now that you've mentioned this, I'll have to look at Ruby and see how it deals with this.

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vanbeast	Ruby is perfect enough :D
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mart	When I talk about it, I normally qualify it as “My perfect programming language”, so that others aren't confused into thinking that I mean “The perfect programming language”, which is of course a fallacy. Ruby can't be my perfect programming language because it doesn't have enough punctuation. `;)`
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vanbeast	heh :) I knew what you meant. I've found Ruby to be awfully close to my perfect programming language. I still put food on the table with perl, but I don't miss the punctuation :)
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evan Haskell allows you to compose functions sorta like Unix pipes: if the output of one matches the input of the other, you don't have to assign it an intermediate name.

Briefly, an anonymous function looks like (\x. x + 1) (this returns whether its argument is greater than one; the backslash is an attempt at producing a lambda). So you can write
map (\x. x + 1) mylist
but you can write it more simply as
map (+ 1) mylist
because + is a two-argument function and you've specified one of the arguments.

There's more on this here:
http://haskell.org/hawiki/PointFreeStyle

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gaal	s/./->/ :)
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evan	Damn, really? I never use that syntax.
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gaal	They use the dot in typeset lambda calculus, where they use a real lambda instead of "\". They also tend to do weird things like bunch up ops in typesetting, which looks bizarre in arrows papers. (As if arrows papers were lacking in bizarritude already.)
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