Don't defunctionalize helpers for class defaults or instance methods #609
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`singletons-th` generates "helper" type families that contain the definitions of class method defaults or instance methods. For example, this: ```hs class C a where m :: a -> b -> a m x _ = x ``` Will be promoted to this: ```hs class PC a where type M (x :: a) (y :: b) :: a type M x y = MHelperSym0 `Apply` x `Apply` y type MHelper :: a -> b -> a type family MHelper x y where MHelper x _ = x type MHelperSym0 :: a ~> b ~> a type MHelperSym1 :: a -> b ~> a ... ``` Generating defunctionalization symbols for `MHelper` is wasteful, however, as we never really _need_ to partially apply `MHelper`. Instead, we can just generate this code: ```hs class PC a where type M (x :: a) (y :: b) :: a type M x y = MHelper x y ``` This takes advantage of the fact that when we apply `MHelper`, we always fully apply it to all of its arguments. This means that we can avoid generating defunctionalization symbols for helper type families altogether, which is a nice optimization. This patch implements that idea, fixing #608 in the process.
…atures As noted in #608, you don't need to involve defunctionalization when promoting class method defaults, as we always know that the promoted versions of the defaults will be fully applied to all their arguments. In fact, there is another situation in which we always know that a promoted type family will be fully applied: the return type of a singled type signature. That it, if you single this definition: ```hs f :: a -> b -> a ``` You currently get this: ```hs sF :: forall a b (x :: a) (y :: b). Sing x -> Sing y -> Sing (FSym0 `Apply` x `Apply` y) ``` But using `FSym0` in the return type of `sF` is silly, however, as we always know that it will be fully applied to its arguments (`x` and `y`). As such, it would be far more direct to instead generate the following: ```hs sF :: forall a b (x :: a) (y :: b). Sing x -> Sing y -> Sing (F x y) ``` No defunctionalization required at all. This doesn't change the behavior of the singled code at all, but it is more direct and will likely require less time to typecheck due to fewer type family indirections being involved. To accomplish this, the `lde_proms` field now maps term-level names to `LetDecProm`s, where a `LetDecProm` consists of the promoted version of the name and the local variables that it closes over. In this context, "promoted version" means `F`, _not_ `FSym0`. Storing the non-defunctionalized name allows us to fully apply `F` whenever it is convenient to do so, and in situations where partial application may be required, we can always convert `F` to `FSym0` before generating the partial application.
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singletons-thgenerates "helper" type families that contain the definitions of class method defaults or instance methods. For example, this:Will be promoted to this:
Generating defunctionalization symbols for
MHelperis wasteful, however, as we never really need to partially applyMHelper. Instead, we can just generate this code:This takes advantage of the fact that when we apply
MHelper, we always fully apply it to all of its arguments. This means that we can avoid generating defunctionalization symbols for helper type families altogether, which is a nice optimization.This patch implements that idea, fixing #608 in the process.