Simon Peyton Jones <simo...@microsoft.com> writes: > Ben, Richard > > rewritableTyVarsOfType is used when deciding whether to split a [WD] > constraint into [W] and [D]. For example, if we are adding > [WD] C a > and there is an inert > [WD] a ~ Int > then we want to split the [WD] into > [W] C a > [D] C a > so that the [D] one can be rewritten and perhaps yield improvement etc. > > There is no point in rewriting coercions or cases in t, because they > won't help (C t) to reduce. That's why rewriteableTyVarsOfType differs > from tyVarsOfType. And indeed the arg of a class C should be a > monotype. > Ahh, I see.
> So rewriteableTyVarsOfType doesn't expect foralls, hence the panic. > But it's seeing a forall in an /equality/ constraint. > > FIRST ISSUE: I'm a bit surprised about that (smacks of impredicative > polymorphism) so the first thing to check is that this is really > supposed to happen in this program. Richard should be able to help. > Indeed this test is meant to fail, just not with a panic. I suspect this means that my solver logic isn't catching requests for polytypes as early as it needs to. > SECOND ISSUE: But regardless, I now realise that shouldSplitWD only > needs to split a [WD] equality if the LHS matches. If we have > [WD] a ~ ty -- work item > [D] a ~ ty2 -- inert > then we want to split the [WD] so we can get [D] ty ~ ty2. But > otherwise there is no point. > > Richard do you agree? > > I'll try that out. > > Meanwhile to get you rolling, you can replace rewritableTyVars with > tyCoVarsOfType and it'll all work, just a bit less efficiently. > Thanks Simon! Indeed that allows things to proceed and in the process, confirms that there is a bug in my solver logic. Cheers, - Ben
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