Re: [Haskell-beginners] Little question about "forall"

forall a. is just what what ScopedTypeVariables uses to ensure that local variables in the function with the same type variables are no longer free, and that they must adhere to the type signature of the encompassing scope.

Hmm, OK, so the extension analizes code/AST and finds a types which are under "forall" and links them (to be exactly the same type at whole scope). Only open question here is: seems that "forall-by-default" is not absilutely true :)

My terminology is terrible, and I've probably made a mistake, but hopefully someone will correct me.

May be I'm not correctly here but Haskell-terminology IMHO is terrible at whole: I found in mail-lists posts about it (that terms are unsuccessful, confusing). As for me, after some Prolog experience I understand what is unification, bound var and not-bound var intuitively - it's clean terminology. So, GHC type-checker for me is some kind of Prolog and if result is True (with all constaints and types relations) then type-checking is passed. But terminology is different and confuse me and I suppose other newbies too: I found already "rigid", "free", "skolem", "bound", etc :-) David, thanks a lot!! === Best regards, Paul

On Thu, Dec 7, 2017 at 10:31 AM, Baa wrote:

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Hello, David!

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But remember that in x, 'a' is not the same as the 'a' in f, so you

Exactly, and I often hit errors like "Can't match rigid type 'M a' with rigid type 'M a1'"! So, the question comes down to "forall" and "ScopedTypeVariables". Did I understand you correctly:

this extension expands type-variable scope to whole function body. But what sort of type vairables? Only under "forall ...", right ?

And another question: type-variables under "forall" - they are rigid? Or free? I can't understand the terminology. I thought early that they are called "rigid", but in documentation of "ScopedTypeVariables" extension they are called "free". IMHO symbol ∀ means "unbound", so "free". But I often hit errors when GHC says something about rigid, so I'm not sure - what is rigid? Bound?

And last: why I need "forall a." in beginning of signature while Haskell assumes "forall" for all type variables by default? Or I'm not right here...

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As to your original code, the only actual bug is that someA :: a is wrong. The actual type of someA is someA :: C a => a, so you could have written

class C a where someA :: a

f :: C a => a -> Int f a = let x :: C a => a x = someA -- absolutely works in 0

But remember that in x, 'a' is not the same as the 'a' in f, so you might as well have written this

f :: C a => a -> Int f a = let x :: C b => b x = someA -- also works in 0

This is how it works. When you write

f :: C a => a -> () f = let x = someA -- without ':: a' ()

The problem is that by not giving an explicit type to x, ghc infers a type and for all intents and purposes it is as though you wrote

f :: C a => a -> () f = let x :: C a1 => a1 x = someA -- also if you had written x = some :: a, it just means you effectively wrote -- let x :: C a1 => a1 -- x = someA :: a -- but 'a1' is not equivalent to 'a', so it errors. ()

And so it says hey wait, a and a1 could be different types. That is why you often see ghc referring to 'a1' or 'a0' in error messages, because when it infers a type it has to give a name to it in order to report on it.

What ScopedTypeVariables does is allow you to specify that for the entire scope of this forall, 'a' in any type signature always refers to the same type.

f :: forall a. C a => a -> () f = undefined where some :: a -- this is guaranteed to be the a from above. some = undefined

somethingelse :: forall a. a -- this is a different a somethingelse = undefined

some :: a -- this toplevel a is of course also different. some = undefined

So you could just go {-# LANGUAGE ScopedTypeVariables #-}

f :: forall a. C a => a -> Int f a = let x :: a -- now the a is the same a as above, so the constraint C a is understood. x = someA in 0

I really love the ScopedTypeVariables extension, and so do a lot of other people. I honestly think it should be on by default because the pitfalls of not having it on are a lot worse than the pitfalls of having it on. The only pitfall I know of to having it on is that if you have a function with tons of lets and / or wheres, you might not notice you had used the type variable 'a' in two different incompatible places.

Hopefully all this makes sense.

On Thu, Dec 7, 2017 at 6:41 AM, Baa wrote:

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Hello everyone!

Suppose I have:

class C a where someA :: a

f :: C a => a -> Int f a = let x = someA :: a in -- BUG!! 0

BUG as I understand I due to `:: a` - this is another `a`, not the same as in `f` singature. But seems that it's the same `a` if `f` is the "method" of some instance. And such bug does not happen if I return it, so my question is how to create such `x` of type `a` in the body of the `f`? How to use `a` anywhere in the body of `f`? Is it possible?

I found a way if I change signature to `forall a. C a => a -> Int`. But there are another questions in the case:

- as I know all such params are under "forall" in Haskell by-default. Seems it's not true? - in method body seems they are under "forall" but in simple functions - not? - i'm not sure what exactly does this "forall", IMHO it unbounds early bound type's variables (parameters) by typechecker, but how `a` can be bound in the just begining of signature (where it occurs first time) ?!

As for me, looks that w/o "forall" all `a`, `b`, `c`, etc in the body of simple functions always are different types. But w/ "forall" Haskell typechecker makes something like (on type-level):

let a = ANY-TYPE in ... here `a` can occur and will be bound to ANY-TYPE

Where am I right and where am I wrong? :)

=== Best regards, Paul _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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