[Haskell-cafe] Non-termination due to context

[Simon Peyton-Jones]

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.

Now, your instance declaration 
	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)

Why? Read the SYB3 paper.

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-----
| From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-bounces at haskell.org] On
| Behalf Of Emil Axelsson
| Sent: 22 January 2010 11:25
| To: Haskell Cafe
| 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
| 
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