Doesn't warn about variable not in scope

Running this works fine

{-# Language GADTs, TypeFamilies, InstanceSigs #-}

data FunC a where
  (:$) :: FunC (a -> b) -> FunC a -> FunC b
  Lam  :: (FunC a -> FunC b) -> FunC (a -> b)
  Add  :: Num a => FunC (a -> a -> a)

class Syntactic a where
  type Internal a
  toFunC   :: a -> FunC (Internal a)
  fromFunC :: FunC (Internal a) -> a

instance Syntactic (FunC a) where
  type Internal (FunC a) = a
  toFunC, fromFunC :: FunC a -> FunC a
  toFunC   ast = ast
  fromFunC ast = ast

instance (Syntactic a, Syntactic b) => Syntactic (a -> b) where
  type Internal (a -> b) = Internal a -> Internal b

  toFunC :: (a -> b) -> FunC (Internal a -> Internal b)
  toFunC f = Lam (toFunC . f . fromFunC)

  fromFunC :: FunC (Internal a -> Internal b) -> (a -> b)
  fromFunC f = fromFunC . (f :$) . toFunC

add :: Num a => FunC a -> FunC a -> FunC a 
add = fromFunC Add

A folklore for infix expressions is writing expr a b as a &expr$ b, if I try that without importing (Data.Function.&) I get

-- tghl.hs:29:23-25: error: …
--     • Couldn't match type ‘Internal t0’ with ‘a0 -> a0 -> a0’
--       Expected type: FunC (Internal t0)
--         Actual type: FunC (a0 -> a0 -> a0)
--       The type variables ‘t0’, ‘a0’ are ambiguous
--     • In the first argument of ‘fromFunC’, namely ‘Add’
--       In the second argument of ‘(&)’, namely ‘fromFunC Add’
--       In the expression: (&) a fromFunC Add
add :: Num a => FunC a -> FunC a -> FunC a 
add a b = a &fromFunC Add$ b

I would expect a Variable not in scope: (&) error.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added GADTs Tbug Trac import labels

closed

Erm, GHC //does// give you a Variable not in scope: (&) error in GHC 8.0.1, 8.0.2 and 8.2.

$ /opt/ghc/8.0.2/bin/ghci Bug2.hs -Wall
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug2.hs, interpreted )

Bug2.hs:29:13: error:
    Variable not in scope: (&) :: FunC a -> t0 -> FunC a -> FunC a

Bug2.hs:29:23: error:
    • Couldn't match type ‘Internal t0’ with ‘a0 -> a0 -> a0’
      Expected type: FunC (Internal t0)
        Actual type: FunC (a0 -> a0 -> a0)
      The type variables ‘t0’, ‘a0’ are ambiguous
    • In the first argument of ‘fromFunC’, namely ‘Add’
      In the second argument of ‘(&)’, namely ‘fromFunC Add’
      In the expression: (&) a fromFunC Add

$ /opt/ghc/8.2.1/bin/ghci Bug2.hs -Wall
GHCi, version 8.2.0.20170321: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug2.hs, interpreted )

Bug2.hs:29:13: error:
    Variable not in scope: (&) :: FunC a -> t0 -> FunC a -> FunC a
   |
29 | add a b = a &fromFunC Add$ b
   |             ^

Bug2.hs:29:23: error:
    • Couldn't match type ‘Internal t0’ with ‘a0 -> a0 -> a0’
      Expected type: FunC (Internal t0)
        Actual type: FunC (a0 -> a0 -> a0)
      The type variables ‘t0’, ‘a0’ are ambiguous
    • In the first argument of ‘fromFunC’, namely ‘Add’
      In the second argument of ‘(&)’, namely ‘fromFunC Add’
      In the expression: (&) a fromFunC Add
   |
29 | add a b = a &fromFunC Add$ b
   |                       ^^^

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedWorksForMe

Ah you need to add

{-# Options_GHC -fdefer-typed-holes #-}

I have it in my .ghci

I'm still not reproducing the error, even with -fdefer-typed-holes.

$ ghci Bug.hs -fdefer-typed-holes
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug.hs, interpreted )

Bug.hs:29:13: error:
    Variable not in scope: (&) :: FunC a -> t0 -> FunC a -> FunC a

Bug.hs:29:23: error:
    • Couldn't match type ‘Internal t0’ with ‘a0 -> a0 -> a0’
      Expected type: FunC (Internal t0)
        Actual type: FunC (a0 -> a0 -> a0)
      The type variables ‘t0’, ‘a0’ are ambiguous
    • In the first argument of ‘fromFunC’, namely ‘Add’
      In the second argument of ‘(&)’, namely ‘fromFunC Add’
      In the expression: (&) a fromFunC Add

added Pnormal label