Question

Possible Duplicate:
Can GHC warn if class instance is a loop?

Consider a type class with two methods which are implementable in terms of each other:

class Num a => Foo a where

  foo :: a

  bar :: a -> a
  bar x = baz x + 1

  baz :: a -> a
  baz x = bar x - 1

Depending on the type, it might be easier to implement bar or baz, or you might want to give implementations of both of them for efficiency reasons.

Now I go somewhere else and make an instance of this class

instance Foo Integer where
  foo = 1

Oops, I forgot to implement either of bar or baz! Never mind, the type system will pick that up for me, won’t it?

C:\path\to\file> ghci Foo.hs
GHCi, version 7.4.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( Foo.hs, interpreted )
Ok, modules loaded: Main.

Hmm, apparently not. Now if I try to use my class

*Main> bar 1
<interactive>: out of memory

Uh oh. Cue hours of painful debugging.

Is there a way to let GHC know that each instance needs to specify at least one of bar or baz?

Answer 1

Unfortunately not. Most libraries that define type classes with defaults will specify a “minimal complete definition”, but they don’t specify that to GHC in a checkable way right now. There’s been some vague talk of implementing a pragma for this, but nothing serious as far as I know.

Note that just checking for mutual recursion isn’t sufficient; mutually recursive default methods might be perfectly valid, e.g. some and many in Alternative.