Question

I’m using lots of different records in a program, with some of them using the same field names, e.g.

data Customer = Customer { ..., foo :: Int, ... }
data Product = Product { ..., foo :: Int, ... }

Now as the accessor function “foo” is defined twice, I get the “Multiple declarations” error. One way to avoid this would be using different modules that are imported fully qualified, or simply renaming the fields (which I don’t want to do).

What is the officially suggested way of dealing with this in Haskell?

Answer 1

This is a very hairy problem. There are several proposals for fixing the record system. On a related note, see TDNR and related discussion on cafe.

Using the currently available language features, I think the best option is defining the two types in two different modules, and doing a qualified import. On top of this, if you want, you can implement some type class machinery.

In Customer.hs

module Customer where
data Customer = Customer { ..., foo :: Int, ... }

In Product.hs

module Product where
data Product = Product { ..., foo :: Int, ... }

While using them, in Third.hs

module Third where

import qualified Customer as C
import qualified Product as P

.. C.foo ..
.. P.foo ..

Yet, I imagine it won’t be too late before you hit the problem about recursively dependent modules.