Status of Haskell'?

Gábor Lehel illissius at
Sun Dec 2 20:56:57 CET 2012

On Sun, Dec 2, 2012 at 7:06 PM, Dan Doel <dan.doel at> wrote:
> This is a significant problem for even some of the more ubiquitous
> extensions. For instance, there are multiple compilers that implement
> RankNTypes, but I would not be surprised at all if programs using that
> extension were not portable across implementations (they're not really even
> portable across GHC versions).
> The problem is that RankNTypes is not just about the fact that you are
> allowed to use such types; every compiler must decide on an inference
> algorithm that incorporates such types while defaulting to Hindley-Milner.
> But, there are several such algorithms, and they have different trade offs
> as far as where annotations must be placed, or even whether certain
> conceivably well-typed terms are type checkable (for instance, GHC used to
> do some level of impredicative instantiation; forall a. a -> a could be
> instantiated to (forall a. a) -> (forall a. a); but this no longer works).
> So, even if we have ubiquitous agreement on the fact that RankNTypes are
> useful, and implementable, we don't have ubiquitous agreement on the
> algorithms for implementing them, and which set of trade offs to make. But
> any standard would have to nail all that down, or else programs won't be
> portable.
> And this is not the only extension for which this kind of thing is an issue.

Out of curiosity, to what degree does MultiParamTypeClasses have this
issue? It seems to me like one of the few extensions which is
straightforward, widely implemented, uncontroversial, and very useful.
For some reason it's been held up by the FDs vs TFs debate, but I
don't see why it has to be. Vanilla MPTCs on the one hand, and MPTCs
together with either FDs or TFs on the other hand, serve different use
cases. If you want a plain type-level relation on types, you use
MPTCs. If you want some types to be determined by others, then you use
either FDs or TFs. If we standardize support for the former, that's
useful, even if we continue to procrastinate on the FDs vs TFs
question. So if the idea is to do yearly incremental updates to the
standard, MPTCs looks like the biggest low-hanging fruit to me.
(Assuming they aren't similarly problematic as RankNTypes.)

Your ship was destroyed in a monadic eruption.

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