Can I have a value constructor named "/""?

Question

I've declared a recursive data type with the following structure:

data Path = GET | POST | Slash Path String

I'd really like to rename that last value constructor to / so that I can infix it in cute expressions like GET /"controller"/"action". However, if I try to do so:

import Prelude hiding ((/))
infixr 5 /
data Path = GET | POST | Path / String

...then I get this:

Path.hs:4:30: parse error on input `/'

Those same three lines compile just fine if I replace / with :/ or any other special character sequence beginning with :.

So, is there any way I can name my value constructor /? I know that I can just name it Slash and then declare a separate function:

(/) :: Path -> String -> Path 
(/) = Slash

...but that won't let me pattern match, as in:

request :: Path -> String
request path = case path of GET /"hello" -> "Hello!"
                            GET /"goodbye" -> "Goodbye!"

Answer 1

Short answer: No.

Long answer: Type classes, type names, and data constructors must begin with either a capital letter or a colon (some of this requires using a language extension). Everything else must begin with a lowercase letter or any other allowed symbol.

Note that type variables, which are normally lowercase identifiers, follow the same rules and do not begin with a colon.

See also the GHC user's guide for enabling type operators. Data constructors are always allowed, I think.

Personally, in your case I'd just use (:/). It doesn't look that bad, and after a while you get used to ignoring the colons. Some people like a trailing colon as well, especially if the data is "symmetric" in some sense.