OK, I'll try to get to the SYB3 paper at some point. For now I'll just
add to my knowledge that UndecidableInstances allows you to create
non-terminating dictionaries in addition to the well-known risk of
making the type checker loop.
Thanks!
/ Emil
Simon Peyton-Jones skrev:
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-boun...@haskell.org
[mailto:haskell-cafe-boun...@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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| Haskell-Cafe mailing list
| Haskell-Cafe@haskell.org
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