the MPTC Dilemma (please solve)

[Simon Peyton-Jones]

I would not say that it's well-specified, no.   What we do know is this:
GHC may loop if you use -fallow-undecidable-instances -- but if it
terminates, the program is well typed and should not "go wrong" at
runtime.

S

| -----Original Message-----
| From: haskell-prime-bounces at haskell.org
[mailto:haskell-prime-bounces at haskell.org] On Behalf Of
| Ashley Yakeley
| Sent: 21 February 2006 20:13
| To: haskell-prime at haskell.org
| Subject: Re: the MPTC Dilemma (please solve)
| 
| Simon Peyton-Jones wrote:
| 
| > Of course -fallow-undecidable-instances still lifts all
restrictions,
| > and then all bets are off.
| 
| Is the behaviour of GHC with -fallow-undecidable-instances (and
| -fcontext-stack) well-understood and specifiable? It is a very useful
| option, as you can join (meet?) classes like this:
| 
|    class P a
|    class Q a
| 
|    class (P a,Q a) => PQ a
|    instance (P a,Q a) => PQ a
| 
| --
| Ashley Yakeley
| 
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