[GHC] #13506: Spurious extra error message due to functional dependencies

31 Mar 2017

      #13506: Spurious extra error message due to functional dependencies
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           Reporter:  gelisam        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 One call site is ill-typed, but GHC reports a type error at every call
 site, not just the problematic one. In the original code, the problem
 occurred with a very common function (`Data.Lens.view`), so the avalanche
 of error messages made it difficult to find the problematic call site.

 {{{#!hs
 {-# LANGUAGE FlexibleInstances, FunctionalDependencies,
 MultiParamTypeClasses #-}
 module Bug where

 class FunDep lista a | lista -> a
 instance FunDep [a] a

 singleton :: FunDep lista a => a -> lista
 singleton _ = undefined

 -- this error is expected:
 --   Couldn't match type 'Char' with '()'
 --   arising from a functional dependency between
 --     constraint 'FunDep [Char] ()' arising from a use of 'singleton'
 --     instance 'FunDep [a] a'
 illTyped :: [Char]
 illTyped = singleton ()

 -- but this one is not:
 --   Couldn't match type '()' with 'Char'
 --     arising from a functional dependency between constraints:
 --       'FunDep [Char] Char' arising from a use of 'singleton' (in
 'wellTyped')
 --       'FunDep [Char] ()' arising from a use of 'singleton' (in
 'illTyped')
 wellTyped :: [Char]
 wellTyped = singleton 'a'
 }}}

 The spurious error disappears if `illTyped` is commented out or moved
 after `wellTyped`.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13506
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