Error message on use of != should suggest use of /=

Currently when one uses != instead of /= it comes up with this error message

        Not in scope: ‘!=’
        Perhaps you meant one of these:
          ‘Map.!’ (imported from Data.Map), ‘>=’ (imported from Prelude),
          ‘==’ (imported from Prelude)

Considering != is very common for unequal in other languages there should be a more helpful hint to use /= instead of or in addition to the current generic symbol error.

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Trac field	Value
Version	7.10.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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changed weight to 5

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This is a good idea, but it immediately leads towards implementing a more general concept, where with any name you can list “common misspellings” that are in scope with the original name, and are consulted if something is not in scope.

How would that be implemented. How about a pragma like this?

  `(!=)` :: a -> a -> Bool
  {-# MISSPELLINGS (!=), (<>) #-}

Naturally, if such a symbol is properly in scope, then the misspelling should be ignored.

Replying to [ticket:12020#comment:119917 nomeata]:

I like it, comments:

Do you have more motivating examples, I have a feeling this would mostly be used for operators.

Would it appear in :info (/=) output? What about :info (!=) whether or not it's defined.

Would it be attached to the definition or definable in a separate module?

Adding a custom error message.

I take it that the first definition was supposed to be (/=) :: a -> a -> Bool:

class Eq a where
  (==) :: a -> a -> Bool
  (/=) :: a -> a -> Bool
  {-# MISSPELLINGS (/=) AS (!=) "‘Not equal to’ is (/=), syntactically emulating the the math symbol ≠" #-}

class Eq a => Bits a where
  (.&.) :: a -> a -> a
  {-# MISSPELLINGS (.&.) AS (&) "Bitwise and is (.&.)." #-}
  (.|.) :: a -> a -> a
  {-# MISSPELLINGS (.|.) AS (|) "Bitwise or is (.|.)." #-}
  ...
  shiftL :: a -> Int -> a
  {-# MISSPELLINGS shiftL AS (<<) "Left shift" #-}

(|) doesn't parse but oh well

ghci> 4 != 10
  
_--_           It looks like you're trying
Ö  Ö      ,- to code. ‘Not equal to’ is (/=), syntactically 
|| |     '   emulating the the math symbol ≠. Try writing
|\_|/          ‘4 /= 10’ instead.
 \_/

This isn't a misspelling, so maybe it could suggest any alternate name:

and :: Foldable t => t Bool -> Bool
{-# MISSPELLINGS and AS every 
  "Determines whether every element of the structure is True."
#-}

or :: Foldable t => t Bool -> Bool
{-# MISSPELLINGS or AS some 
  "Determines whether some element of the structure is True." 
#-}

mentioned in issue #12121

added Pnormal label