Motivation

If a user is unfamiliar with the restriction on : , the message can lead them pretty far astray.

Example 1:

Prelude> let (:=) :: Show a => String -> a -> String

<interactive>:3:5:error:
    Invalid type signature: (:=) :: ...
    Should be of form <variable> :: <type>

Example 2:

> let (:=) a b = undefined

<interactive>:2:5: error:
    Not in scope: data constructor ‘:=’
    Perhaps you meant one of these:
      variable ‘>=’ (imported from Prelude),
      variable ‘==’ (imported from Prelude),
      variable ‘/=’ (imported from Prelude)
> data Foo a b = a := b
> 

Proposal

Less a proposal, and more a hypothesis. Example 2 above could maybe look like

> let (:=) a b = undefined

<interactive>:2:5: error:
    Syntax restriction: ':' is reserved for data constructor operators

I say "more a hypothesis" because I'm not sure that message is precise enough.