Typechecking fails for parallel monad comprehensions with polymorphic let (GHC 7.10.3 through 8.6.3)
{-# LANGUAGE MonadComprehensions, ParallelListComp #-}
module Foo where
foo xs ys = [ (f y True, f x 'c')
| let f _ z = z, x <- xs
| y <- ys ]This fails with
Foo.hs:5:52: error:
* Cannot instantiate unification variable `t0'
with a type involving foralls: forall t2 t3. t3 -> t2 -> t2
GHC doesn't yet support impredicative polymorphism
* In a stmt of a monad comprehension:
let f _ z = z, x <- xs | y <- ys
In the expression:
[(f y True, f x 'c') | let f _ z = z, x <- xs | y <- ys]
In an equation for `foo':
foo xs ys
= [(f y True, f x 'c') | let f _ z = z, x <- xs | y <- ys]NB: ApplicativeDo has a related problem: the implementation is quite different and has the effect of monomorphising the let-bound variable.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.10.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
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| Related | |
| Blocking | |
| CC | |
| Operating system | |
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