Correct the Applicative & MonadFix instances for ErrorC #228
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It was previously incorrect in that `throwError e <*> a` would diverge when `a` diverged, but `throwError e \`ap\` a` would not.
robrix
changed the title
robrix
commented
Sep 28, 2019
| newtype ErrorC e m a = ErrorC { runErrorC :: m (Either e a) } | ||
| deriving (Functor) | ||
| newtype ErrorC e m a = ErrorC { runErrorC :: ExceptT e m a } | ||
| deriving (Applicative, Functor, Monad, Fail.MonadFail, MonadFix, MonadIO, MonadTrans) |
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Defining ErrorC atop ExceptT simultaneously fixes the unlawful Applicative instance and allows us to remove a great number of hand-written instances.
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It also fixes the divergent MonadFix instance.
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| instance Alternative m => Alternative (ErrorC e m) where | ||
| empty = ErrorC empty | ||
| instance (Alternative m, Monad m) => Alternative (ErrorC e m) where |
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We still define our own Alternative and MonadPlus instances in order to retain our existing semantics instead of adopting ExceptT’s (which handles them itself using a Monoid constraint on e instead of passing them along to m).
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Can we leave this as documentation in the file? Haddocks on those instances, maybe?
robrix
changed the title
patrickt
approved these changes
Sep 29, 2019
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It was previously incorrect in that
throwError e <*> awould diverge whenadiverged, butap (throwError e) awould not.