Heterogenous collections

This page is a very hasty and ad-hoc summary of a common discussion on the haskell-cafe list. If you find it hard to read, please complain there and somebody hopefully will come to help.

The problem

Does some kind of collection of objects with different types in Haskell exist? Obviously, tuples are an example, but they have a fixed length. To compare tuples vs lists:

Tuples	Lists
Heterogeneous	Homogeneous
Fixed length (per tuple type)	Variable length
Always finite	May be unending

However, the need is for heterogeneous and non-fixed length. When one is used to Java, with its loose typing of collections,not having this immediately and easily available seems strange. As an example, the need is for something like LinkedList<Object> from java.

Algebraic datatypes

If the number of types to cover is fixed, then the problem can be solved by a list of data types such as

data T =
     ConsInt    Int
   | ConsString String
   | ConsChar   Char

or:

data Object = IntObject Int | StringObject String

objectString :: Object -> String
objectString (IntObject v) = show v
objectString (StringObject v) = v

main = mapM_ (putStrLn . objectString) [(IntObject 7), (StringObject "eight")]

This is a very basic solution, and often preferable. Limitations: You have to type-switch all the time if you want to do anything with the objects in the List, and the collections are clumsy to extend by new types.

HLists, OOHaskell, type-level programming

This is the cleanest solution, but very advanced and a little restrictive. Read these two articles:

• http://homepages.cwi.nl/~ralf/HList/
• http://homepages.cwi.nl/~ralf/OOHaskell/

There is also some related material here:

• Extensible record

Existential types

Depending on needs and comfort level with fancier types, the existential approach to ADTs might solve the problem. The following code is a demonstration you can cut-and-paste-and-run.

This is example akin to upcasting in Java to an interface that lets you print things. That way you know how to print every object (or do whatever else it is you need to do) in the list. Beware: there is no safe downcasting (that's what Typeable would be for); that would likely be more than you need.

There are other ways to do this with existentials (e.g. bounded existentials), but this is what came out of my head when I read your post. Existentials seems to be the "Haskellish" way to reduce a heterogenous list to a collection of objects with common operations. HList would be the Haskellish way for more static and flexible assurances.

{-# OPTIONS -fglasgow-exts #-}

module Test where

data PrintPackage = forall a . PrintPackage a (a -> String)

instance Show PrintPackage where
   show (PrintPackage val showMethod) = showMethod val

list = [ PrintPackage 3 show
      , PrintPackage "string" show
      , PrintPackage 3.4 show
      ]

main = print list