[Haskell-beginners] Re: Closure

[Heinrich Apfelmus]

Daniel Bastos wrote:
>
>> So it sounds correct to say that a closure is a function that brings
>> an environment with it, such as variables defined outside of it. 
>>
>> With this ability, we can construct functions on the fly because a
>> function can return a closure which is amended and, say, returned
>> again another closure more fully specified. 
> 
> Hello. This was actually a request for comments. Though I didn't say
> it. Does that sound correct? Any comments? Thanks much.

Yes, that's pretty much correct.

The simplest example of a closure is indeed

   foo = add 3

where

   add = \x y -> x + y

Reduction to weak head normal form yields

   foo = let x = 3 in \y -> x + y

which means that  foo  is a function  \y -> x + y  paired with the value
of the free variable  x .


Note that closures are an implementation detail. From a semantic point
of view,  add 3  can readily be understood as an ordinary function.



Regards,
apfelmus

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