[Haskell-cafe] Importing Control.Arrow changes inferred type of (m
>>= f) x in ghci
Dan Weston
westondan at imageworks.com
Mon Jul 27 23:58:29 EDT 2009
The following inferred type has a constraint that can be trivially
satisfied, but isn't:
Control.Monad> :t \ (m,f,x) -> (m >>= f) x
\ (m,f,x) -> (m >>= f) x
:: forall t a b. (Monad ((->) t)) => (t -> a, a -> t -> b, t) -> b
-- In Control.Monad there is forall t. instance Monad ((->) t),
-- so why is the vacuous Monad constraint still there?
-- Nor can I remove it with a type annotation:
Control.Monad> :t \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a
-> t -> b, t) -> b
<interactive>:1:13:
Inferred type is less polymorphic than expected
[snip]
-- Then if I just import a module:
Control.Monad> :m + Control.Arrow
-- Now, the Monad ((-> t) constraint disappears:
Control.Monad Control.Arrow> :t \ (m,f,x) -> (m >>= f) x
\ (m,f,x) -> (m >>= f) x
:: forall t a b. (t -> a, a -> t -> b, t) -> b
-- Although it still will not accept an explicit type annotation:
Control.Monad Control.Arrow> :t \ (m,f,x) -> (m >>= f) x :: forall t a
b. (t -> a, a -> t -> b, t) -> b
<interactive>:1:13:
Inferred type is less polymorphic than expected
[snip]
Is there some explicit kind annotation I can/should give to get the most
general type?
Dan
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