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common_errors.md

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Common Errors

There are certain classes of errors when using fused-effects that GHC is not particularly good at explaining. This document is an attempt to enumerate and explicate the things that can go wrong when using or extending this library. (It is also very much a work in progress.)

I'm getting kind errors when implementing an Algebra instance!

Given an effect datatype that doesn’t use the m parameter:

data Fail m k where
  Fail :: String -> Fail m a

newtype FailC m a = FailC { runFailC :: m (Either String a) }

Declaring an Algebra instance will fail:

instance Algebra sig m
      => Algebra (Fail :+: sig) (FailC m) where…
• Expected kind ‘(* -> *) -> * -> *’,
    but ‘Fail :+: sig’ has kind ‘* -> * -> *’
• In the first argument of ‘Algebra’, namely ‘(Fail :+: sig)’
  In the instance declaration for
    ‘Algebra (Fail :+: sig) (FailC m)

This is because the m parameter to Fail is inferred to be of kind *: though Algebra expects an m of kind * -> *, m is never referenced in the definition of Fail, so GHC makes an understandable but incorrect inference. An explicit kind annotation on m fixes the problem.

data Fail (m :: * -> *) k where
  Fail :: String -> Fail m a