"Data Constructor not in Scope" when defining an operator function starting with :

[TotallyNotChase]

Defining a function starting with : is invalid according to the parsing rules:

f :< g = f . g

However, the error message here could see some improvement. It currently shows:

error: Not in scope: data constructor ‘:<’

Perhaps it should instead explicitly note that only data constructors may start with colons, and function names must not.

error: Function name may not start with a colon (':')

Or:

error: Only data constructor names may start with a colon (':')

[noughtmare]

One challenge is that this could would compile correctly if you added some definitions above it:

import Preluding hiding (.)

data A = () :< ()

(.) :: a -> b -> A
(.) _ _ = () :< ()

f :< g = f . g

This example is contrived because I didn't try very hard to make it realistic, but this at least shows that the original message isn't always wrong.

I think it is best to add something to the error message like:

error: Not in scope: data constructor ‘:<’
Perhaps you wanted to define an operator, but operator names can't start with a colon ':'

[googleson78]

Crucially/for example, this can be relied upon in servant-client when you're not using named routes in order to extract a specific function from the functions generated based on an API.

[goldfirere]

I like @noughtmare's suggestion of adding a little informative bit to the error message teaching users about the capitalization rules. (: is a capital symbol, while the others are lowercase, evidently.)