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by mjs 6072 days ago
Well, is there any real different between a first-class function and a closure that closes over nothing?

Is it possible for a language to support closures without first-class functions? How common is it to have first-class functions without closures? (I think Python sort of had this arrangement, but I think this is fixed now.)
2 comments
andreyf 6072 days ago
A closure is just a (function, bindings) pair, so Python certainly has closures. The problem with Python is that there is no syntax for assigning to variables declared in an arbitrary scope, because there are no explicit declarations. As far as I understand, implicit local variables are what causes the problems in Python, not anything about closures, really. This causes other problems, too, such as those PG described here [1], under the heading "Implicit local variables conflict with macros".

1. http://www.paulgraham.com/arclessons.html

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micheles 6072 days ago
Python 3 has read-write closures. Check out the nonlocal declaration.
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andreyf 6072 days ago
The nonlocal statement causes the listed identifiers to refer to previously bound variables in the nearest enclosing scope. [1]

While certainly a big improvement, we still can't access variables in arbitrary scopes, just local, nearest non-local, and global. To me, this seems a significantly more complex way of specifying variable scope than just declaring it explicitly.

1. http://docs.python.org/dev/3.0/reference/simple_stmts.html#t...

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mjs 6072 days ago
Interesting. Thanks.
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blasdel 6072 days ago
Haskell does not semantically have closures at all, because functions have only their arguments -- no scope to close over.

Lexically, there's where clauses to scope function definitions, but that gets desugared very early, and is really not the same thing at all anyway (the name binding is completely static).

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distortion 6072 days ago
Of course Haskell has closures, there's always a scope to close over:

f x = (\y -> x + y)

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blasdel 6072 days ago
That's still just high-level lexical sugar - the 'scope' is not captured, just rewritten. After desugaring, there is no scope left, even lexically.

It's completely different from a 'closure' in a language that has variables.

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