mistaken qualified infix syntax could have a nicer error message

[Isaac Dupree]

Consider the expression Prelude.(+) ; we get Not in scope: data constructor `Prelude'. Unfortunately this is valid code if the module-referencing name also is the name of an in-scope data constructor. In the common case that it isn't, at least (a cursory inspection doesn't find any such exports in the libraries, e.g. Data.Map doesn't export a data-constructor Map), a message to the effect that infix operators must be qualified like (Prelude.+) instead, would be nice. (hierarchical module names probably make this a little more complicated somehow)

[Yuras]

It can be pretty valid expression. An example:

Prelude> data Prelude a = Prelude a
Prelude> :t Prelude.(+)
Prelude.(+) :: Num a => a -> Prelude (a -> a)
Prelude> 

The ticket seems invalid for me.

[simonpj]

Yes, in Haskell Prelude.(+) means (Prelude . (+)), that is the composition of the function Prelude and the function (+).

There's a Haskell Prime proposal to change this:  http://hackage.haskell.org/trac/haskell-prime/wiki/QualifiedOperators

Till then I don't see how we can improve the error message.

Simon