[Haskell-cafe] Problem with tuple with one element constrained

[Sergiu Ivanov]

Hello Tyson,

Thus quoth  Tyson Whitehead  at 19:41 on Tue, Apr 18 2017:
>
> I've been unable to get the following (simplified) code to be accepted
> by GHCi (8.0.1 and 8.0.2)
>
> Prelude> let (x, y) = (1, return 2)
[...]
> and I don't seem to be able to provide a type signatures that GHCi is
> okay with

GHCi handles polymorphic types of interactive expressions somewhat
differently from the polymorphic types of expressions loaded from
files. (I'd be happy if someone could reproduce the explanation which I
saw a couple times but which I cannot find any more.)


Anyway, I put the following in tuple.hs:

func :: Monad m => (Int, m Int)
func = (1, return 2)

Then, ghci tuple.hs swallows the file no problem and allows me to do

*Main> func :: (Int, Maybe Int)
(1, Just 2)


(I have GHCi 8.0.2.)

--
Sergiu
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