Why this Either-monad code does not type check?
- by pf_miles
instance Monad (Either a) where
return = Left
fail = Right
Left x >>= f = f x
Right x >>= _ = Right x
this code frag in 'baby.hs' caused the horrible compilation error:
Prelude> :l baby
[1 of 1] Compiling Main ( baby.hs, interpreted )
baby.hs:2:18:
Couldn't match expected type `a1' against inferred type `a'
`a1' is a rigid type variable bound by
the type signature for `return' at <no location info>
`a' is a rigid type variable bound by
the instance declaration at baby.hs:1:23
In the expression: Left
In the definition of `return': return = Left
In the instance declaration for `Monad (Either a)'
baby.hs:3:16:
Couldn't match expected type `[Char]' against inferred type `a1'
`a1' is a rigid type variable bound by
the type signature for `fail' at <no location info>
Expected type: String
Inferred type: a1
In the expression: Right
In the definition of `fail': fail = Right
baby.hs:4:26:
Couldn't match expected type `a1' against inferred type `a'
`a1' is a rigid type variable bound by
the type signature for `>>=' at <no location info>
`a' is a rigid type variable bound by
the instance declaration at baby.hs:1:23
In the first argument of `f', namely `x'
In the expression: f x
In the definition of `>>=': Left x >>= f = f x
baby.hs:5:31:
Couldn't match expected type `b' against inferred type `a'
`b' is a rigid type variable bound by
the type signature for `>>=' at <no location info>
`a' is a rigid type variable bound by
the instance declaration at baby.hs:1:23
In the first argument of `Right', namely `x'
In the expression: Right x
In the definition of `>>=': Right x >>= _ = Right x
Failed, modules loaded: none.
why this happen? and how could I make this code compile ?
thanks for any help~
