I've gotten this sort of error several times, which mysteriously
disappears
when I add more functions to the code:
storeError.hs:13:38:
Couldn't match expected type `a' (a rigid variable)
against inferred type `String'
`a' is bound by the type signature for `throwError'
at <no location info>
Expected type: a
Inferred type: String
In the first argument of `return', namely `msg'
In the call (return msg)
(This is GHCi.) The code is below. The type variable a can't be
bound to
String, obviously, but a relative novice like myself has no idea
why. Can
someone tell me?
Thanks,
-Rod
--
module Store where
import Control.Monad.Error
import Control.Concurrent.STM
data StoreError = Default String
instance Error StoreError where
noMsg = Default "Store error"
strMsg = Default
instance MonadError StoreError STM where
throwError (Default msg) = return msg
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