Type synonym with context in pattern synonym
pattern :: (Num a, Eq a) => a
pattern Fiskur = 42
unlike
type Diskur a = Num a => a
fiskur :: Num a => a
fiskur = 42
we cannot write
type Fugl a = (Num a, Eq a) => a
pattern Fiskur :: Fugl a
pattern Fiskur = 42
-- Exp.hs:54:18-19: error: …
-- • No instance for (Eq a) arising from a pattern
-- Possible fix:
-- add (Eq a) to the context of
-- the type signature for pattern synonym ‘Fiskur’:
-- Fugl a
-- • In the pattern: 42
-- In the declaration for pattern synonym ‘Fiskur’
-- Compilation failed.
Given that the story of pattern synonym contexts is more complicated than usual patterns given provided constraints, I know there are other issues (#11524 (closed), #10928 (closed)).
For fun why not type synonym with provided and required constraints:
data T a where
MkT :: (Show b) => a -> b -> T a
f1 :: (Eq a, Num a) => T a -> String
f1 (MkT 42 x) = show x
type MKNUMPAT a b = (Num a, Eq a) => (Show b) => b -> T a
-- pattern ExNumPat :: (Num a, Eq a) => (Show b) => b -> T a
pattern ExNumPat :: MKNUMPAT a b
pattern ExNumPat x = MkT 42 x
What would that mean in a regular type signature? CE := CP + CR
where EXNUMPAT a b
means the same as EXNUMPAT' a b
? (#8581)
type MKNUMPAT' a b = (Num a, Eq a, Show b) => b -> T a
-- This works:
mkNumPat :: MKNUMPAT a b
mkNumPat = ExNumPat
Trac metadata
Trac field | Value |
---|---|
Version | 8.1 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |