Haskell get types of Data Constructor

Question

I was wondering if given a constructor, such as:

data UserType = User
  { username :: String
  , password :: String
  } -- deriving whatever necessary

What the easiest way is for me to get something on the lines of [("username", String), ("password", String)], short of just manually writing it. Now for this specific example it is fine to just write it but for a complex database model with lots of different fields it would be pretty annoying.

So far I have looked through Typeable and Data but so far the closest thing I have found is:

user = User "admin" "pass"
constrFields (toConstr user)

But that doesn't tell me the types, it just returns ["username", "password"] and it also requires that I create an instance of User.

Answer 1

I just knocked out a function using Data.Typeable that lets you turn a constructor into a list of the TypeReps of its arguments. In conjunction with the constrFields you found you can zip them together to get your desired result:

{-# LANGUAGE DeriveDataTypeable #-}

module Foo where
import Data.Typeable
import Data.Typeable.Internal(funTc)

getConsArguments c = go (typeOf c)
   where go x = let (con, rest) = splitTyConApp x    
                in if con == funTc         
                   then case rest of (c:cs:[]) -> c : go cs
                                     _ -> error "arrows always take two arguments"
                   else []                      

given data Foo = Foo {a :: String, b :: Int} deriving Typeable, we get

*> getConsArguments Foo
[[Char],Int]

As one would hope.

---

On how to get the field names without using a populated data type value itself, here is a solution:

constrFields . head . dataTypeConstrs $ dataTypeOf (undefined :: Foo)