[Builtin] Define 'caseList' in terms of 'chooseList' #4119
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kwxm
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Oct 17, 2021
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Thanks! This all looks good. I plugged this into my benchmarking code, and using the new folds is about 20% faster for summing up lists than the versions using ifThenElse. That's better than I was expecting, and it'll give us a fairer comparison against the compiled-from-Haskell versions.
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/benchmark plutus-benchmark:validation |
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The main change is replacing
```haskell
data BuiltinRuntime val
= BuiltinCostedResult ExBudgetStream ~(BuiltinResult val)
| <...>
```
with
```haskell
data BuiltinRuntime val
= BuiltinCostedResult ExBudgetStream ~(BuiltinResult (HeadSpine val))
| <...>
```
where `HeadSpine` is a fancy way of saying `NonEmpty`:
```haskell
-- | A non-empty spine. Isomorphic to 'NonEmpty', except is strict and is defined as a single
-- recursive data type.
data Spine a
= SpineLast a
| SpineCons a (Spine a)
-- | The head-spine form of an iterated application. Provides O(1) access to the head of the
-- application. Isomorphic to @nonempty@, except is strict and the no-spine case is made a separate
-- constructor for performance reasons (it only takes a single pattern match to access the head when
-- there's no spine this way, while otherwise we'd also need to match on the spine to ensure that
-- it's empty -- and the no-spine case is by far the most common one, hence we want to optimize it).
data HeadSpine a
= HeadOnly a
| HeadSpine a (Spine a)
```
(we define a separate type, because we want strictness, and you don't see any bangs, because it's in a module with `StrictData` enabled).
The idea is that a builtin application can return a function applied to a bunch of arguments, which is exactly what we need to be able to express `caseList`
```haskell
caseList xs0 f z = case xs0 of
[] -> z
x:xs -> f x xs
```
as a builtin:
```haskell
-- | Take a function and a list of arguments and apply the former to the latter.
headSpine :: Opaque val asToB -> [val] -> Opaque (HeadSpine val) b
headSpine (Opaque f) = Opaque . \case
[] -> HeadOnly f
x0 : xs ->
-- It's critical to use 'foldr' here, so that deforestation kicks in.
-- See Note [Definition of foldl'] in "GHC.List" and related Notes around for an explanation
-- of the trick.
HeadSpine f $ foldr (\x2 r x1 -> SpineCons x1 $ r x2) SpineLast xs x0
instance uni ~ DefaultUni => ToBuiltinMeaning uni DefaultFun where
<...>
toBuiltinMeaning _ver CaseList =
let caseListDenotation
:: Opaque val (LastArg a b)
-> Opaque val (a -> [a] -> b)
-> SomeConstant uni [a]
-> BuiltinResult (Opaque (HeadSpine val) b)
caseListDenotation z f (SomeConstant (Some (ValueOf uniListA xs0))) = do
case uniListA of
DefaultUniList uniA -> pure $ case xs0 of
[] -> headSpine z [] -- [1]
x : xs -> headSpine f [fromValueOf uniA x, fromValueOf uniListA xs] -- [2]
_ ->
-- See Note [Structural vs operational errors within builtins].
throwing _StructuralUnliftingError "Expected a list but got something else"
{-# INLINE caseListDenotation #-}
in makeBuiltinMeaning
caseListDenotation
(runCostingFunThreeArguments . unimplementedCostingFun)
```
Being able to express [1] (representing `z`) and [2] (representing `f x xs`) is precisely what this PR enables.
Adding support for the new functionality to the CEK machine is trivial. All we need is a way to push a `Spine` of arguments onto the context:
```haskell
-- | Push arguments onto the stack. The first argument will be the most recent entry.
pushArgs
:: Spine (CekValue uni fun ann)
-> Context uni fun ann
-> Context uni fun ann
pushArgs args ctx = foldr FrameAwaitFunValue ctx args
```
and a `HeadSpine` version of `returnCek`:
```haskell
-- | Evaluate a 'HeadSpine' by pushing the arguments (if any) onto the stack and proceeding with
-- the returning phase of the CEK machine.
returnCekHeadSpine
:: Context uni fun ann
-> HeadSpine (CekValue uni fun ann)
-> CekM uni fun s (Term NamedDeBruijn uni fun ())
returnCekHeadSpine ctx (HeadOnly x) = returnCek ctx x
returnCekHeadSpine ctx (HeadSpine f xs) = returnCek (pushArgs xs ctx) f
```
Then replacing
```haskell
BuiltinSuccess x ->
returnCek ctx x
```
with
```haskell
BuiltinSuccess fXs ->
returnCekHeadSpine ctx fXs
```
(and similarly for `BuiltinSuccessWithLogs`) will do the trick.
We used to define `caseList` in terms of `IfThenElse`, `NullList` and either `HeadList` or `TailList` depending on the result of `NullList`, i.e. three builtin calls in the worst and in the best case. Then we introduced `ChooseList`, which replaced both `IfThenElse` and `NullList` in `caseList` thus bringing total amount of builtin calls down to 2 in all cases. This turned out to have a [substantial](#4119 (review)) impact on performance. This PR allows us to bring total number of builtin calls per `caseList` invokation down to 1 -- the `CaseList` builtin itself.
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The main change is replacing
```haskell
data BuiltinRuntime val
= BuiltinCostedResult ExBudgetStream ~(BuiltinResult val)
| <...>
```
with
```haskell
data BuiltinRuntime val
= BuiltinCostedResult ExBudgetStream ~(BuiltinResult (HeadSpine val))
| <...>
```
where `HeadSpine` is a fancy way of saying `NonEmpty`:
```haskell
-- | A non-empty spine. Isomorphic to 'NonEmpty', except is strict and is defined as a single
-- recursive data type.
data Spine a
= SpineLast a
| SpineCons a (Spine a)
-- | The head-spine form of an iterated application. Provides O(1) access to the head of the
-- application. Isomorphic to @nonempty@, except is strict and the no-spine case is made a separate
-- constructor for performance reasons (it only takes a single pattern match to access the head when
-- there's no spine this way, while otherwise we'd also need to match on the spine to ensure that
-- it's empty -- and the no-spine case is by far the most common one, hence we want to optimize it).
data HeadSpine a
= HeadOnly a
| HeadSpine a (Spine a)
```
(we define a separate type, because we want strictness, and you don't see any bangs, because it's in a module with `StrictData` enabled).
The idea is that a builtin application can return a function applied to a bunch of arguments, which is exactly what we need to be able to express `caseList`
```haskell
caseList xs0 f z = case xs0 of
[] -> z
x:xs -> f x xs
```
as a builtin:
```haskell
-- | Take a function and a list of arguments and apply the former to the latter.
headSpine :: Opaque val asToB -> [val] -> Opaque (HeadSpine val) b
headSpine (Opaque f) = Opaque . \case
[] -> HeadOnly f
x0 : xs ->
-- It's critical to use 'foldr' here, so that deforestation kicks in.
-- See Note [Definition of foldl'] in "GHC.List" and related Notes around for an explanation
-- of the trick.
HeadSpine f $ foldr (\x2 r x1 -> SpineCons x1 $ r x2) SpineLast xs x0
instance uni ~ DefaultUni => ToBuiltinMeaning uni DefaultFun where
<...>
toBuiltinMeaning _ver CaseList =
let caseListDenotation
:: Opaque val (LastArg a b)
-> Opaque val (a -> [a] -> b)
-> SomeConstant uni [a]
-> BuiltinResult (Opaque (HeadSpine val) b)
caseListDenotation z f (SomeConstant (Some (ValueOf uniListA xs0))) = do
case uniListA of
DefaultUniList uniA -> pure $ case xs0 of
[] -> headSpine z [] -- [1]
x : xs -> headSpine f [fromValueOf uniA x, fromValueOf uniListA xs] -- [2]
_ ->
-- See Note [Structural vs operational errors within builtins].
throwing _StructuralUnliftingError "Expected a list but got something else"
{-# INLINE caseListDenotation #-}
in makeBuiltinMeaning
caseListDenotation
(runCostingFunThreeArguments . unimplementedCostingFun)
```
Being able to express [1] (representing `z`) and [2] (representing `f x xs`) is precisely what this PR enables.
Adding support for the new functionality to the CEK machine is trivial. All we need is a way to push a `Spine` of arguments onto the context:
```haskell
-- | Push arguments onto the stack. The first argument will be the most recent entry.
pushArgs
:: Spine (CekValue uni fun ann)
-> Context uni fun ann
-> Context uni fun ann
pushArgs args ctx = foldr FrameAwaitFunValue ctx args
```
and a `HeadSpine` version of `returnCek`:
```haskell
-- | Evaluate a 'HeadSpine' by pushing the arguments (if any) onto the stack and proceeding with
-- the returning phase of the CEK machine.
returnCekHeadSpine
:: Context uni fun ann
-> HeadSpine (CekValue uni fun ann)
-> CekM uni fun s (Term NamedDeBruijn uni fun ())
returnCekHeadSpine ctx (HeadOnly x) = returnCek ctx x
returnCekHeadSpine ctx (HeadSpine f xs) = returnCek (pushArgs xs ctx) f
```
Then replacing
```haskell
BuiltinSuccess x ->
returnCek ctx x
```
with
```haskell
BuiltinSuccess fXs ->
returnCekHeadSpine ctx fXs
```
(and similarly for `BuiltinSuccessWithLogs`) will do the trick.
We used to define `caseList` in terms of `IfThenElse`, `NullList` and either `HeadList` or `TailList` depending on the result of `NullList`, i.e. three builtin calls in the worst and in the best case. Then we introduced `ChooseList`, which replaced both `IfThenElse` and `NullList` in `caseList` thus bringing total amount of builtin calls down to 2 in all cases. This turned out to have a [substantial](IntersectMBO#4119 (review)) impact on performance. This PR allows us to bring total number of builtin calls per `caseList` invokation down to 1 -- the `CaseList` builtin itself.
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As per @kwxm's request.