# [Haskell-cafe] Non-termination due to context

Simon Peyton-Jones simonpj at microsoft.com
Fri Jan 22 08:19:17 EST 2010

```It's a feature!

You have
* B is a superclass of A
* Eq is a superclass of B

So every A dictionary has a B dictionary inside it, and every B dictionary has an Eq dictionary inside it.

instance (A a, Eq a) => B a
says "if you give me an A dictionary and an Eq dictionary, I'll make you a B dictionary".

Now, 'test' needs a (B Int) dictionary.  To get one, we need an (A Int) dictionary and an (Eq Int) dictionary.  But

when solving these sub-problems, GHC assumes that you
have in hand a solution to the original problem, this case (B Int)

OK so now you see the problem: we can solve the (A Int) and (Eq Int) sub-problems by selection from the (B Int) dictionary.

Still, I confess that I have not fully grokked the relationship between the SYB3-style recursion stuff and the question of superclasses.   So I will think about your example some more, thank you.

Meanwhile, it's clear that you are on thin ice.

Simon

| -----Original Message-----
| Behalf Of Emil Axelsson
| Sent: 22 January 2010 11:25
| Subject: [Haskell-cafe] Non-termination due to context
|
| Hello all!
|
| Consider the following program:
|
| > {-# LANGUAGE FlexibleInstances, OverlappingInstances, UndecidableInstances #-}
| >
| > class B a => A a
| >
| > instance A Int
| >
| > class Eq a => B a
| >
| > instance (A a, Eq a) => B a
| >
| > eq :: B a => a -> a -> Bool
| > eq = (==)
| >
| > test = 1 `eq` (2::Int)
|
| (This is a condensed version of a much larger program that I've been
| debugging.)
|
| It compiles just fine, but `test` doesn't terminate (GHCi 6.10.4). If I
| change the context `B a` to `Eq a` for the function `eq`, it terminates.
|
| Although I don't know all the details of the class system, it seems
| unintuitive that I can make a program non-terminating just by changing
| the context of a function (regardless of UndecidableInstances etc.).
|
| Is this a bug or a feature?
|
| / Emil
|
| _______________________________________________