Pattern synonym fails with (Exp Bool) but works with (t ~ Bool => Exp t)
The following compiles just fine:
data Exp a where
B :: Bool -> Exp Bool
pattern Tru = B True
pr :: Exp a -> String
pr Tru = "true"
but add a signature to Tru
…
pattern Tru :: Exp Bool
pattern Tru = B True
…and it fails to compile
% ghci -ignore-dot-ghci /tmp/Bug.hs
GHCi, version 7.10.0.20150316: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( /tmp/Bug.hs, interpreted )
/tmp/Bug.hs:11:4:
Couldn't match type ‘a’ with ‘Bool’
‘a’ is a rigid type variable bound by
the type signature for pr :: Exp a -> String at /tmp/Bug.hs:10:7
Expected type: Exp a
Actual type: Exp Bool
Relevant bindings include
pr :: Exp a -> String (bound at /tmp/Bug.hs:11:1)
In the pattern: Tru
In an equation for ‘pr’: pr Tru = "true"
Failed, modules loaded: none.
Checking the inferred type of pattern Tru = B True
I get
ghci> :i Tru
pattern Tru :: t ~ Bool => Exp t
which works as expected
pattern Tru :: t ~ Bool => Exp t
pattern Tru = B True
pr :: Exp a -> String
pr Tru = "true"
Trac metadata
Trac field | Value |
---|---|
Version | 7.10.1-rc1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |