[Haskell-cafe] Problem with tuple with one element constrained
Tyson Whitehead
twhitehead at gmail.com
Tue Apr 18 19:41:22 UTC 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)
<interactive>:1:5: error:
• Could not deduce (Monad m0)
from the context: Num t
bound by the inferred type for ‘x’:
Num t => t
at <interactive>:1:5-24
The type variable ‘m0’ is ambiguous
• When checking that the inferred type
x :: forall a (m :: * -> *) t. (Num a, Num t, Monad m) => t
is as general as its inferred signature
x :: forall t. Num t => t
and I don't seem to be able to provide a type signatures that GHCi is okay with
Prelude> let { x :: Int; y :: Monad m => m Int; (x, y) = (1, return 2) }
<interactive>:5:40: error:
• Ambiguous type variable ‘m0’
prevents the constraint ‘(Monad m0)’ from being solved.
• When checking that the inferred type
x :: forall a (m :: * -> *) t (m1 :: * -> *).
(Num a, Num t, Monad m) =>
t
is as general as its signature
x :: Int
Prelude> let (x, y) = (1, return 2) :: (Int, Monad m => m Int)
<interactive>:4:31: error:
• Illegal qualified type: Monad m => m Int
GHC doesn't yet support impredicative polymorphism
• In an expression type signature: (Int, Monad m => m Int)
In the expression: (1, return 2) :: (Int, Monad m => m Int)
In a pattern binding:
(x, y) = (1, return 2) :: (Int, Monad m => m Int)
Prelude> let (x,y) = (1,return 2) :: Monad m => (Int, m Int)
<interactive>:7:5: error:
• Ambiguous type variable ‘m0’
prevents the constraint ‘(Monad m0)’ from being solved.
• When checking that the inferred type
x :: forall (m :: * -> *). Monad m => Int
is as general as its inferred signature
x :: Int
It seems the constraint on y leaks to x where it isn't well formed? Any help and/or explanations would be greatly appreciated. In my actual code the assignment is from the return value of a function so I can't just split it into two separate statements.
Thanks! -Tyson
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