Make singling preserve type variable order in type signatures #408
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goldfirere
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Sep 5, 2019
This patch tweaks `singType` (and its sister function `singCtor`) to produce type signatures that preserve the order in which type variables appear. For instance, if one writes this: ```hs const2 :: forall b a. a -> b -> a ``` Then its singled counterpart will now quantify `b` and `a` in the same order: ```hs sConst2 :: forall b a (x :: a) (y :: b). Sing x -> Sing y -> Sing (Const2 x y) ``` This is important if one wants to combine `sConst2` with visible type applications, as this means that the meaning of `@T1 @T2` in `const @t1 @T2` will be preserved when one writes `sConst2 @t1 @T2`. This also paves the way for #378, which seeks to fully support promoting/singling type applications in the TH machinery. Most of the interesting parts of this patch are described in `Note [Preserve the order of type variables during singling]` in `D.S.Single.Type`, so start that is a good starting point when looking at this patch for the first time. Fixes chunk (1) in #378 (comment), but there are still other parts of #378 that remain to be fixed.
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This patch tweaks
singType(and its sister functionsingCtor) to produce type signatures that preserve the order in which type variables appear. For instance, if one writes this:Then its singled counterpart will now quantify
bandain the same order:This is important if one wants to combine
sConst2with visible type applications, as this means that the meaning of@T1 @T2inconst @T1 @T2will be preserved when one writessConst2 @T1 @T2. This also paves the way for #378, which seeks to fully support promoting/singling type applications in the TH machinery.Most of the interesting parts of this patch are described in
Note [Preserve the order of type variables during singling]inD.S.Single.Type, so start that is a good starting point when looking at this patch for the first time.Fixes chunk (1) in #378 (comment), but there are still other parts of #378 that remain to be fixed.