View Patterns affect typechecking in an unpredictable manner
Copying the text from my StackOverflow question.
{-# LANGUAGE ViewPatterns #-}
import Data.Text (Text)
import qualified Data.Text as T
import qualified Data.Vector.Unboxed as UV
import qualified Data.Vector.Generic as V
bar :: Int -> UV.Vector Char -> (Text, Text)
bar i v = (t_pre, t_post)
where
f = T.pack . V.toList
(f -> t_pre, f -> t_post) = V.splitAt i v
This gives an unexpected type error (tested with GHC 8.4.3), concerning ambiguous type variables and f
being out of scope.
• Ambiguous type variable ‘v0’ arising from a use of ‘V.toList’
prevents the constraint ‘(V.Vector v0 Char)’ from being solved.
Relevant bindings include
f :: v0 Char -> Text (bound at Weird.hs:11:5)
Probable fix: use a type annotation to specify what ‘v0’ should be.
These potential instances exist:
instance V.Vector UV.Vector Char
-- Defined in ‘Data.Vector.Unboxed.Base’
...plus one instance involving out-of-scope types
instance primitive-0.6.3.0:Data.Primitive.Types.Prim a =>
V.Vector Data.Vector.Primitive.Vector a
-- Defined in ‘Data.Vector.Primitive’
• In the second argument of ‘(.)’, namely ‘V.toList’
In the expression: T.pack . V.toList
In an equation for ‘f’: f = T.pack . V.toList
|
11 | f = T.pack . V.toList
| ^^^^^^^^
Weird.hs:13:6: error:
Variable not in scope: f :: UV.Vector Char -> t
|
13 | (f -> t_pre, f -> t_post) = V.splitAt i v
| ^
Weird.hs:13:18: error:
Variable not in scope: f :: UV.Vector Char -> t1
|
13 | (f -> t_pre, f -> t_post) = V.splitAt i v
| ^
Some of the other answers on SO have done some digging (e.g. enabling FlexibleContexts / NoMonomorphismRestriction doesn't solve the issue). I haven't yet figured out how to minimize the example further to remove the dependency on Vector...
Is this a compiler bug or a documentation bug?
Trac metadata
Trac field | Value |
---|---|
Version | 8.4.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |