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More synify-ing improvements #931
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b6a12c0
Better rendering of unboxed sums/tuples
harpocrates 7d455c7
Handle promoted tuples in 'synifyType'
harpocrates 7ace2a6
Accept test cases
harpocrates b4526dc
Properly synify promoted list types
harpocrates 307f33a
Only look at visible types when synifying a 'HsListTy'
harpocrates ffcf6c2
Various improvements to synifying Types
harpocrates 421fc78
Deduce proper order for type variables
harpocrates 1c8d152
Infer kinds in pattern syns based on kind of TyCon
harpocrates 05974a3
Synify associated type defaults
harpocrates 0aaeba6
Extend variables in scope when synifying
harpocrates 25d3db5
Add example of 'tyCoVarsOfTypeList's behaviour
harpocrates 3dcf333
Move 'forall's left on pattern synonym sigs
harpocrates dece147
Fix return kind of polykinded H98 types
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| ><head | ||
| ><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" | ||
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| >UnboxedStuff</title | ||
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| {-# LANGUAGE UnboxedSums, UnboxedTuples #-} | ||
| module UnboxedStuff where | ||
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| data X | ||
| data Y | ||
| data Z | ||
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| -- * Unboxed type constructors | ||
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| unboxedUnit :: (# #) -> (# #) | ||
| unboxedUnit = undefined | ||
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| unboxedTuple :: (# X, Y #) -> (# X, Y, Z #) | ||
| unboxedTuple = undefined | ||
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| unboxedSum :: (# X | Y #) -> (# X | Y | Z #) | ||
| unboxedSum = undefined | ||
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| \haddockmoduleheading{UnboxedStuff} | ||
| \label{module:UnboxedStuff} | ||
| \haddockbeginheader | ||
| {\haddockverb\begin{verbatim} | ||
| module UnboxedStuff ( | ||
| X, Y, Z, unboxedUnit, unboxedTuple, unboxedSum | ||
| ) where\end{verbatim}} | ||
| \haddockendheader | ||
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| \begin{haddockdesc} | ||
| \item[\begin{tabular}{@{}l} | ||
| data\ X | ||
| \end{tabular}] | ||
| \end{haddockdesc} | ||
| \begin{haddockdesc} | ||
| \item[\begin{tabular}{@{}l} | ||
| data\ Y | ||
| \end{tabular}] | ||
| \end{haddockdesc} | ||
| \begin{haddockdesc} | ||
| \item[\begin{tabular}{@{}l} | ||
| data\ Z | ||
| \end{tabular}] | ||
| \end{haddockdesc} | ||
| \section{Unboxed type constructors} | ||
| \begin{haddockdesc} | ||
| \item[ | ||
| unboxedUnit\ ::\ ({\char '43}\ {\char '43})\ ->\ ({\char '43}\ {\char '43}) | ||
| ] | ||
| \item[ | ||
| unboxedTuple\ ::\ ({\char '43}\ X,\ Y\ {\char '43})\ ->\ ({\char '43}\ X,\ Y,\ Z\ {\char '43}) | ||
| ] | ||
| \item[ | ||
| unboxedSum\ ::\ ({\char '43}\ X\ |\ Y\ {\char '43})\ ->\ ({\char '43}\ X\ |\ Y\ |\ Z\ {\char '43}) | ||
| ] | ||
| \end{haddockdesc} |
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I must be missing some context here. Why would we ever want to retrieve free type variables in reverse order?
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We want them in order. Unfortunately, that's not what GHC does.
unionFVstarts by running the secondFVon the accumulator, then passing that to the firstFV. That means it adds variables from the second argument to thein_scopeset before adding variables from the first argument. All this is bad news if the first and second arguments share some variables.Here is a snippet demonstrating the somewhat surprising behaviour:
My hack here is to traverse everything in reverse order, then reverse the list of type variables at the end.
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Oh dear, that's absolutely terrible. I really thought that
tyCoVarsOfTypesWellScopedwould be enough to accomplish this task, but your counterexample shows that I was mistaken.Still, I'm incredibly bothered by the fact that we have to duplicate so much machinery just to solve this problem. I've sent an email to the ghc-devs list asking about this, so I'm hoping I can get some clarification on this point.
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Based on Simon's response here, it looks like the
FV-returning functions really don't guarantee any particular order (only that it's deterministic), which is a huge bummer.Ideally, we'd rewrite
tyCoFVsOfTypeand friends in such a way that they always return things in left-to-right order. Your hacked version accomplishes this, but at the cost of reversing the order at the very end, which feels wasteful. Is there a way to accomplish the same thing with only one pass?There was a problem hiding this comment.
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In GHC: yes. We'd need to change
unitFVto append to the var list. In order to keep that operation efficient, that means swapping occurrences of[Var]inFVfor difference lists ofVar. It looks like theFVcode has already been carefully performance tuned, so we'd have to be very careful there.In Haddock: not really, short of porting over most of
FV(and then making the changes we'd otherwise make in GHC).What frustrates me the most is not the
reverse: it's having to rely on what is accidental behaviour in GHC.There was a problem hiding this comment.
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Ugh, this is all so nasty.
Ultimately, I think the
orderedFVshack is probably the way to go for now. I'd only request that you expand the documentation for it to include an example of whytyCoVarsOfTypeWellScopeddoesn't give the behavior you desire (e.g., your example in #931 (comment)).Ideally, we'd investigate making the
FVfunctions in GHC return things in a reliable left-to-right order. But as you've noted, they're quite performance intensive, so I can understand your reservations about making things slower by changing the underlying data structures involved.There was a problem hiding this comment.
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I've added an example illustrating GHC's behaviour (I didn't know about
alphaTyVarandbetaTyVar- thanks!).I agree we should eventually look into making
FVreturn things in a reliable left-to-right order. I'm a bit swamped at the moment, but I'll try to circle back when all of this Hi Haddock stuff settles down.