[Haskell-cafe] Position of a constraint in a function's signature

[Brandon Allbery]

There's a number of weirdnesses currently that basically fall out of the
fact that the kinds Constraint and * are treated identically internally.
See https://ghc.haskell.org/trac/ghc/ticket/11715 and stay tuned; it's not
going to be fixed for 8.2 but probably will for 8.4 --- unless more bugs
come crawling out of the woodwork, as has been happening a lot if you look
at the ticket history. :)

On Mon, May 15, 2017 at 5:09 PM, Al Zohali <zohl at fmap.me> wrote:

> Dear Cafe!
>
> I've encountered quite strange behaviour in GHC (v8.0.2) and I would
> like to ask you to give me a hint.
>
> Suppose we have the following functions:
> ----
> foo :: (Show a) => Int -> a -> [String]
> foo n = replicate n . show
>
> bar :: Int -> (Show a) => a -> [String]
> bar n = replicate n . show
>
> baz :: Int -> a -> (Show a) => [String]
> baz n = replicate n . show
> ----
>
> This won't compile, and that is ok. But if we add `RankNTypes`
> extension, this will compile and (:t) will give us the same signature
> for all three functions.
>
> There are two things I cannot get:
>
> 1) Why do this even compile?
> I saw constraints being defined either in the beginning of a signature
> or right after `forall` expression. I thought that it was a rule (or
> convention), but it's not. Is this way of declaring constraints (in
> the middle of a signature) discouraged or can be considered as a bug?
>
> 2) Even if this was supposed to be so, why was the constraint in `baz`
> hoisted to the top?
> There are at least two ways to interpret that signature:
> ----
> baz :: Int -> a -> forall a. (Show a) => [String]
> baz :: forall a. (Show a) => Int -> a -> [String]
> ----
> Is there any reason why the second one was chosen?
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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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