Haskell: Defaulting Rules

Posted on December 31, 2014 by Alexey Shmalko
Tags: haskell

I’ve signed for the Haskell-cafe mailing list to learn something new a day ago. The first letter I received was asking which defaulting proposals are actively pursued. I’d knew nothing about defaulting and why it should be improved, so that’s what I learned today.

What’s defaulting and default declaration?

A problem inherent with Haskell type system is the possibility of an ambiguous type. It arises when compiler can’t determine the concrete type of subexpression from source code. The easiest example of that is:

let x = read "..." in show x

There is no way for a compiler to infer the type of x. Any a that is an instance of both Show and Read would satisfy the constraints.

The default way to resolve ambiguities is to specify the type explicitly. For example:

let x = read "..." in show (x :: Int)

Ambiguities in the class Num are most common, so Haskell provides another way to resolve them—defaulting. If the compiler can’t figure out which instance of Num it should choose, it will pick the default one. You’ll see the following warning with -Wall if that happens:

Warning: Defaulting the following constraint(s) to type ‘Integer’

You may assume the default instance for Num is Integer. Almost guessed! I’ll tell you the answer a bit later.

Haskell provides a way to select what types compiler should consider as default for Numdefault declaration.

To specify the list of types, you can write (where all types are instances of Num):

default (type1, ..., typeN)

The compiler will choose the first one that satisfies the constraints.

Only one default declaration is permitted per module, and its effect is limited to that module. If no default declaration is given in a module it is assumed to be:

default (Integer, Double)

To turn off defaulting, specify empty list:

default ()

Unfortunately, even if you explicitly specify default types for a module, the compiler will warn you about defaulting (which can be a pain in the ass with -Werror).

So what’s wrong with current rules and what are proposals?

Problem 1. User-defined classes

The most significant problem is that defaulting rules are applied to Num class only. Specifying defaulting rules for user-defined classes would be nice.

Two proposals address this issue. The first one suggests specifying class name before the list of defaults. The problem arises when type variable is constrained by two classes with different defaults. For example:

default A (Int, String)
default B (String, Int)
(A t, B t) => t

What type should compiler choose? I don’t know.

The next proposal avoids this kind of problems simply by permitting only one default type per class, rather than a list. In that case, there is only two option: all default types match or they don’t.

default A Int
default B String
(A t, B t) => t

Now it’s a failure for sure. And the main cons of this proposal are backward compatibility and lack of a way to disable defaulting. The latter is easily fixable by allowing omission of the default type in default declaration.

Problem 2. Scoping

A default clause applies only to the module containing the declaration. Defaults can be neither exported nor imported.

For me, this problem arises only if problem 1 is resolved. Exporting defaulting rules for you own typeclass in any way is a nice feature to have.

Currently, the single proposal addressing this issue is to make all defaulting rules global.

I believe it’s not the best option. I’d like to have local defaulting rules with the possibility of explicit exporting.

Problem 3. Defaulting

The last proposal is the most radical one: remove defaulting altogether. It’s motivated by the fact that it’s generally agreed that defaulting, in its current form, at least, is a wart on the language.

It also proposes to change signature of (^) and introduce genericPower (because the ^ is the most frequent cause of defaulting):

(^)          :: (Num a) => a -> Int -> a
genericPower :: (Num a, Integral b) => a -> b -> a

It also expect interactive environments to continue default values.

Personally, I’d like to write sum [1..100] and get 5050 instead of an error.

Further reading