[Haskell-cafe] Surprising lack of generalisation

[Richard Eisenberg]

This is the effect of -XMonoLocalBinds, which is implied by -XTypeFamilies (and also by -XGADTs). See https://downloads.haskell.org/ghc/latest/docs/html/users_guide/exts/let_generalisation.html <https://downloads.haskell.org/ghc/latest/docs/html/users_guide/exts/let_generalisation.html>.

Happy to give more background -- let me know if that link doesn't answer your question.

Richard

> On Feb 10, 2021, at 6:31 AM, Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
> 
> Dear all,
> 
> I don't understand why the type of pBD defined in the where clause of
> pFG cannot be inferred to a general type in the presence of
> TypeFamilies.  In particular I don't understand why nonetheless the
> type of pBD definied in the where clause of pF (only slightly simpler)
> *can* be inferred.
> 
> Can anyone explain?
> 
> Thanks
> 
> Tom
> 
> 
> 
> {-# LANGUAGE TypeFamilies #-}
> 
> -- This code came up in the context of writing a parser, but that's
> -- not terribly important
> 
> import Prelude hiding ((<$>))
> 
> data B = B
> 
> data D f = F     f
>         | GF AP f
>         | DF AM f
> 
> data AM = AM
> data AP = AP
> 
> pB :: Parser B
> pB = Parser
> 
> -- Works fine
> pF :: Parser (D B)
> pF = pBD GF AP <|> pBD DF AM
>  where pBD f p = f p <$> pB
> 
> -- Shows the (presumably) inferred type for pBD
> pFWithType :: Parser (D B)
> pFWithType = pBD GF AP <|> pBD DF AM
>  where pBD :: (t -> B -> b) -> t -> Parser b
>        pBD f p = f p <$> pB
> 
> -- One would hope this type would be inferred too
> pFGWithType :: Parser B -> Parser (D B)
> pFGWithType pBArg = pBD GF AP <|> pBD DF AM
>  where pBD :: (t -> B -> b) -> t -> Parser b
>        pBD f p = f p <$> pBArg
> 
> -- But omitting it leads to a type error if TypeFamilies is enabled.
> -- There is no error if TypeFamilies is not enabled.
> pFG :: Parser B -> Parser (D B)
> pFG pBArg = pBD GF AP <|> pBD DF AM
>  where pBD f p = f p <$> pBArg
> 
> 
> -- The specifics of the parser don't matter
> data Parser a = Parser
> 
> (<|>) :: Parser a -> Parser a -> Parser a
> (<|>) _ _ = Parser
> 
> (<$>) :: (a -> b) -> Parser a -> Parser b
> (<$>) _ _ = Parser
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