Opened 7 years ago

Last modified 6 years ago

#5858 closed bug

type inference of an OverloadedString for a class instance with type parameters — at Initial Version

Reported by: GregWeber Owned by:
Priority: normal Milestone:
Component: Compiler Version: 7.4.1
Keywords: Cc: greg@…
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


We have some code in Yesod:

class RedirectUrl master a where
    -- | Converts the value to the URL and a list of query-string parameters.
    toTextUrl :: a -> GHandler sub master Text

instance t ~ Text => RedirectUrl master (Route master, [(t, t)]) where
    toTextUrl (u, ps) = do
        r <- getUrlRenderParams
        return $ r u ps

When I use it in my application, I am required to give an annotation to the overloaded strings. If I don't:

redirect $ (SearchR, [("foo", "bar")])

I end up with this error message:

    No instance for (RedirectUrl Search (Route Search, [(t0, t1)]))
      arising from a use of `redirect'
    Possible fix:
      add an instance declaration for
      (RedirectUrl Search (Route Search, [(t0, t1)]))
    In the expression: redirect
    In the expression: redirect $ (SearchR, [("foo", "bar")])
    In an equation for `getFoodsr23R':
        getFoodsr23R foodId = redirect $ (SearchR, [("foo", "bar")])

I would be ok with having to type annotate if instead of the compiler suggesting I declare an entire new instance the compiler instead suggested that I annotate my overloaded strings.

However, in trying to reproduce this program in a simpler setting, it seems to normally perform the OverloadedStrings inference without any issue. This works just fine:

{-# LANGUAGE OverloadedStrings, NoMonomorphismRestriction, FlexibleInstances, GADTs #-}
module InferOverloaded where
import Data.Text

class InferOverloaded a where
  infer :: a -> a

data Data = Data String

instance t ~ Text => InferOverloaded (Data, [(t,t)]) where
  infer = id

foo = infer (Data "data", [("overloaded", "strings")])

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