#2239: lack of improvement/reduction with TFs
----------------------------------------------+-----------------------------
Reporter: claus | Owner: simonpj
Type: bug | Status: closed
Priority: low | Milestone: 7.0.1
Component: Compiler (Type checker) | Version: 6.12.3
Resolution: fixed | Keywords: TF FD
Testcase: indexed-types/should_fail/T2239 | Blockedby:
Difficulty: Unknown | Os:
Unknown/Multiple
Blocking: | Architecture:
Unknown/Multiple
Failure: None/Unknown |
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Changes (by simonpj):
* status: new => closed
* testcase: => indexed-types/should_fail/T2239
* resolution: => fixed
Comment:
The new typechecker is happy with all these except `complexFD` and
`complexTF` which fail in the same way
{{{
T2239.hs:43:13:
Couldn't match expected type `b -> b'
with actual type `forall b1. MyEq b1 Bool => b1 -> b1'
Expected type: (forall b1. MyEq b1 Bool => b1 -> b1) -> b -> b
Actual type: (forall b1. MyEq b1 Bool => b1 -> b1)
-> forall b1. MyEq b1 Bool => b1 -> b1
In the expression:
id ::
(forall b. MyEq b Bool => b -> b)
-> (forall b. MyEq b Bool => b -> b)
In an equation for `complexFD':
complexFD
= id ::
(forall b. MyEq b Bool => b -> b)
-> (forall b. MyEq b Bool => b -> b)
T2239.hs:47:13:
Couldn't match expected type `b -> b'
with actual type `forall b1. b1 ~ Bool => b1 -> b1'
Expected type: (forall b1. b1 ~ Bool => b1 -> b1) -> b -> b
Actual type: (forall b1. b1 ~ Bool => b1 -> b1)
-> forall b1. b1 ~ Bool => b1 -> b1
In the expression:
id ::
(forall b. b ~ Bool => b -> b) -> (forall b. b ~ Bool => b -> b)
In an equation for `complexTF':
complexTF
= id ::
(forall b. b ~ Bool => b -> b) -> (forall b. b ~ Bool => b
-> b)
}}}
To make them work you need to instantiate `id` at a polytype, which
involves impredicativity.
I've added the examples as a regression test though.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2239#comment:10>
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