Restricted Types and Infinite Loops

Simon Peyton-Jones simonpj at
Fri Feb 11 06:42:05 EST 2005


You've found an interesting case. 

First, you are skating on thin ice here.  GHC's ability to build
recursive dictionaries is quite experimental, and you are relying on it

But you're right: it "should" work.  I can see why it isn't but I have
not got it clear enough in my head to know the best way to fix it.
Still less do I have a formal story about what should type-check
(without loops) and what should not.

So this is going to continue to fail in 6.4, but it's on my list to look


| -----Original Message-----
| From: glasgow-haskell-users-bounces at
| bounces at] On Behalf Of Simon David Foster
| Sent: 27 January 2005 14:11
| To: GHC Users
| Subject: Restricted Types and Infinite Loops
| Hi,
| (I've attached the full code for this problem)
| First I'll explain the problem description, I have two class ClassA
| ClassB, the former has two parameters and the latter has one. The
| parameter of ClassA is constrained by ClassB.
| class ClassB a where
| class ClassB b => ClassA a b where
| Because I wish to effectively pass the context of ClassA around, I
| to create a pair of dictionary types (as in Restricted Data Types in
| Haskell, Hughes 99), one to represent ClassA (DictClassA) and one to
| represent ClassB (DictClassB). DictClassA also contains a term of type
| DictClassB since ClassA is a subclass of ClassB. I should then be able
| to call all the functions of ClassB via the appropriate term of
| DictClassA, like so (assuming we want to use func2);
| *Test> func2D (classBD (dict::DictClassA Int String)) "hello"
| "bye"
| So far so good, but now suppose I want Class A to have the further
| constraint
| class (Data (DictClassA a) b, ClassB b) => ClassA a b where
| (so as to make ClassA a subclass of Data)
| If we now try and do
| *Test> func2D (classBD (dict::DictClassA Int String)) "hello"
| We go into an infinite loop. Why? The expression still type-checks ok
| and I can't see what it is trying to do. All the functions of ClassA
| be accessed ok, but not ClassB.
| *Test> funcD ((dict::DictClassA Int String)) "hello" 5
| "hello"
| Is it something to do with ClassB only having one parameter?
| I'm running GHC 20041231.
| -Si.
| --
| Simon David Foster <s.d.f at>

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