module Effects import Language.Reflection import public Effect.Default import Data.Vect import public Data.List --- Effectful computations are described as algebraic data types that --- explain how an effect is interpreted in some underlying context. %access export -- ----------------------------------------------------------------- [ Effects ] ||| The Effect type describes effectful computations. ||| ||| This type is parameterised by: ||| + The return type of the computation. ||| + The input resource. ||| + The computation to run on the resource given the return value. public export Effect : Type Effect = (x : Type) -> Type -> (x -> Type) -> Type ||| The `EFFECT` Data type describes how to promote the Effect ||| description into a concrete effect. public export %error_reverse data EFFECT : Type where MkEff : Type -> Effect -> EFFECT -- 'sig' gives the signature for an effect. There are four versions -- depending on whether there is no resource needed, -- no state change, a non-dependent change, -- or a dependent change. These are easily disambiguated by type. namespace NoResourceEffect public export sig : Effect -> Type -> Type sig e r = e r () (\v => ()) namespace NoUpdateEffect public export sig : Effect -> (ret : Type) -> (resource : Type) -> Type sig e r e_in = e r e_in (\v => e_in) namespace UpdateEffect public export sig : Effect -> (ret : Type) -> (res_in : Type) -> (res_out : Type) -> Type sig e r e_in e_out = e r e_in (\v => e_out) namespace DepUpdateEffect public export sig : Effect -> (ret : Type) -> (res_in : Type) -> (res_out : ret -> Type) -> Type sig e r e_in e_out = e r e_in e_out ||| Handler interfaces describe how an effect `e` is translated to the ||| underlying computation context `m` for execution. public export interface Handler (e : Effect) (m : Type -> Type) where ||| How to handle the effect. ||| ||| @ r The resource being handled. ||| @ eff The effect to be applied. ||| @ k The continuation to pass the result of the effect covering handle : (r : res) -> (eff : e t res resk) -> (k : ((x : t) -> resk x -> m a)) -> m a ||| Get the resource type (handy at the REPL to find out about an effect) resourceType : EFFECT -> Type resourceType (MkEff t e) = t -- --------------------------------------------------------- [ Syntactic Sugar ] -- A bit of syntactic sugar ('syntax' is not very flexible so we only go -- up to a small number of parameters...) -- No state transition syntax "{" [inst] "}" [eff] = eff inst (\result => inst) -- The state transition is dependent on a result `b`, a bound variable. syntax "{" [inst] "==>" "{" {b} "}" [outst] "}" [eff] = eff inst (\b => outst) --- A simple state transition syntax "{" [inst] "==>" [outst] "}" [eff] = eff inst (\result => outst) -- --------------------------------------- [ Properties and Proof Construction ] public export data SubElem : a -> List a -> Type where Z : SubElem a (a :: as) S : SubElem a as -> SubElem a (b :: as) public export data SubList : List a -> List a -> Type where SubNil : SubList [] xs InList : SubElem x ys -> SubList xs ys -> SubList (x :: xs) ys Uninhabited (SubElem x []) where uninhabited Z impossible uninhabited (S _) impossible -- Some useful hints for proof construction in polymorphic programs %hint public export total dropFirst : SubList xs ys -> SubList xs (x :: ys) dropFirst SubNil = SubNil dropFirst (InList el sub) = InList (S el) (dropFirst sub) %hint public export total subListId : (xs : List a) -> SubList xs xs subListId [] = SubNil subListId (x :: xs) = InList Z (dropFirst (subListId xs)) public export total inSuffix : SubElem x ys -> SubList xs ys -> SubElem x (zs ++ ys) inSuffix {zs = []} el sub = el inSuffix {zs = (x :: xs)} el sub = S (inSuffix el sub) %hint public export total dropPrefix : SubList xs ys -> SubList xs (zs ++ ys) dropPrefix SubNil = SubNil dropPrefix (InList el sub) = InList (inSuffix el sub) (dropPrefix sub) public export total inPrefix : SubElem x ys -> SubList xs ys -> SubElem x (ys ++ zs) inPrefix {zs = []} {ys} el sub = rewrite appendNilRightNeutral ys in el inPrefix {zs = (x :: xs)} Z sub = Z inPrefix {zs = (x :: xs)} (S y) sub = S (inPrefix y SubNil) %hint public export total dropSuffix : SubList xs ys -> SubList xs (ys ++ zs) dropSuffix SubNil = SubNil dropSuffix (InList el sub) = InList (inPrefix el sub) (dropSuffix sub) namespace Env public export data Env : (m : Type -> Type) -> List EFFECT -> Type where Nil : Env m Nil (::) : Handler eff m => a -> Env m xs -> Env m (MkEff a eff :: xs) total envElem : SubElem x xs -> Env m xs -> Env m [x] envElem Z (x :: xs) = [x] envElem (S k) (x :: xs) = envElem k xs ||| make an environment corresponding to a sub-list total dropEnv : Env m ys -> SubList xs ys -> Env m xs dropEnv [] SubNil = [] dropEnv [] (InList idx rest) = absurd idx dropEnv (y::ys) SubNil = [] dropEnv e@(y::ys) (InList idx rest) = let [x] = envElem idx e in x :: dropEnv e rest public export total updateAt : (idx : SubElem x' xs) -> (a:Type) -> List EFFECT -> List EFFECT updateAt Z a [] = [] updateAt Z a ((MkEff b eff) :: xs) = (MkEff a eff) :: xs updateAt (S k) a [] = [] updateAt (S k) a (x :: xs) = x :: updateAt k a xs public export total updateWith : (ys' : List EFFECT) -> (xs : List EFFECT) -> SubList ys xs -> List EFFECT updateWith [] xs sl = xs updateWith (y :: ys) xs SubNil = xs updateWith ((MkEff a f) :: ys) xs (InList idx rest) = updateAt idx a (updateWith ys xs rest) public export total replaceEnvAt : (x : a) -> (idx : SubElem x' xs) -> Env m ys -> Env m (updateAt idx a ys) replaceEnvAt x Z [] = [] replaceEnvAt x Z (y :: ys) = x :: ys replaceEnvAt x (S k) [] = [] replaceEnvAt x (S k) (y :: ys) = y :: replaceEnvAt x k ys ||| Put things back, replacing old with new in the sub-environment public export total rebuildEnv : {ys':List EFFECT} -> Env m ys' -> (prf : SubList ys xs) -> Env m xs -> Env m (updateWith ys' xs prf) rebuildEnv [] SubNil env = env rebuildEnv (x :: xs) SubNil env = env rebuildEnv [] (InList w s) env = env rebuildEnv (x :: xs) (InList idx rest) env = replaceEnvAt x idx (rebuildEnv xs rest env) -- -------------------------------------------------- [ The Effect EDSL itself ] public export total updateResTy : (val : t) -> (xs : List EFFECT) -> with List Elem (MkEff a e) xs -> e t a b -> List EFFECT updateResTy {b} val (MkEff a e :: xs) Here n = (MkEff (b val) e) :: xs updateResTy val (x :: xs) (There p) n = x :: updateResTy val xs p n infix 5 :::, :-, := public export data LRes : lbl -> Type -> Type where (:=) : (x : lbl) -> res -> LRes x res public export (:::) : lbl -> EFFECT -> EFFECT (:::) {lbl} x (MkEff r e) = MkEff (LRes x r) e using (lbl : Type) implementation Default a => Default (LRes lbl a) where default = lbl := default private unlabel : {l : ty} -> Env m [l ::: x] -> Env m [x] unlabel {m} {x = MkEff a eff} [l := v] = [v] private relabel : (l : ty) -> Env m xs -> Env m (map (\x => l ::: x) xs) relabel {xs = []} l [] = [] relabel {xs = (MkEff a e :: xs)} l (v :: vs) = (l := v) :: relabel l vs -- ------------------------------------------------- [ The Language of Effects ] ||| Definition of a language of effectful programs. ||| ||| @ x The return type of the result. ||| @ es The list of allowed side-effects. ||| @ ce Function to compute a new list of allowed side-effects. public export data EffM : (m : Type -> Type) -> (x : Type) -> (es : List EFFECT) -> (ce : x -> List EFFECT) -> Type where Value : (val : a) -> EffM m a (xs val) xs EBind : EffM m a xs xs' -> ((val : a) -> EffM m b (xs' val) xs'') -> EffM m b xs xs'' CallP : (prf : with List Elem (MkEff a e) xs) -> (eff : e t a b) -> EffM m t xs (\v => updateResTy v xs prf eff) LiftP : (prf : SubList ys xs) -> EffM m t ys ys' -> EffM m t xs (\v => updateWith (ys' v) xs prf) New : Handler e' m => (e : EFFECT) -> resTy -> {auto prf : e = MkEff resTy e'} -> EffM m t (e :: es) (\v => e :: es) -> EffM m t es (\v => es) (:-) : (l : ty) -> EffM m t [x] xs' -> -- [x] (\v => xs) -> EffM m t [l ::: x] (\v => map (l :::) (xs' v)) -- Some type synonyms, so we don't always have to write EffM in full namespace SimpleEff -- Simple effects, no updates public export Eff : (x : Type) -> (es : List EFFECT) -> Type Eff x es = {m : Type -> Type} -> EffM m x es (\v => es) public export EffT : (m : Type -> Type) -> (x : Type) -> (es : List EFFECT) -> Type EffT m x es = EffM m x es (\v => es) namespace TransEff -- Dependent effects, updates not dependent on result public export Eff : (x : Type) -> (es : List EFFECT) -> (ce : List EFFECT) -> Type Eff x es ce = {m : Type -> Type} -> EffM m x es (\_ => ce) public export EffT : (m : Type -> Type) -> (x : Type) -> (es : List EFFECT) -> (ce : List EFFECT) -> Type EffT m x es ce = EffM m x es (\_ => ce) namespace DepEff -- Dependent effects, updates dependent on result public export Eff : (x : Type) -> (es : List EFFECT) -> (ce : x -> List EFFECT) -> Type Eff x es ce = {m : Type -> Type} -> EffM m x es ce public export EffT : (m : Type -> Type) -> (x : Type) -> (es : List EFFECT) -> (ce : x -> List EFFECT) -> Type EffT m x es ce = EffM m x es ce %no_implicit (>>=) : EffM m a xs xs' -> ((val : a) -> EffM m b (xs' val) xs'') -> EffM m b xs xs'' (>>=) = EBind -- namespace SimpleBind -- (>>=) : Eff m a xs (\v => xs) -> -- ((val : a) -> Eff m b xs xs') -> Eff m b xs xs' -- (>>=) = EBind ||| Run a subprogram which results in an effect state the same as the input. staticEff : EffM m a xs (\v => xs) -> EffM m a xs (\v => xs) staticEff = id ||| Explicitly give the expected set of result effects for an effectful ||| operation. toEff : .(xs' : List EFFECT) -> EffM m a xs (\v => xs') -> EffM m a xs (\v => xs') toEff xs' = id infixl 2 <*> pure : a -> EffM m a xs (\v => xs) pure = Value return : a -> EffM m a xs (\v => xs) return = pure %deprecate Effects.return "Please use `pure`." pureM : (val : a) -> EffM m a (xs val) xs pureM = Value (<*>) : EffM m (a -> b) xs (\v => xs) -> EffM m a xs (\v => xs) -> EffM m b xs (\v => xs) (<*>) prog v = do fn <- prog arg <- v pure (fn arg) (<$>) : (a -> b) -> EffM m a xs (\v => xs) -> EffM m b xs (\v => xs) (<$>) f v = pure f <*> v (*>) : EffM m a xs (\v => xs) -> EffM m b xs (\v => xs) -> EffM m b xs (\v => xs) a *> b = do a b new : Handler e' m => (e : EFFECT) -> resTy -> {auto prf : e = MkEff resTy e'} -> EffM m t (e :: es) (\v => e :: es) -> EffM m t es (\v => es) new = New -- ---------------------------------------------------------- [ an interpreter ] private execEff : Env m xs -> (p : with List Elem (MkEff res e) xs) -> (eff : e a res resk) -> ((v : a) -> Env m (updateResTy v xs p eff) -> m t) -> m t execEff (val :: env) Here eff' k = handle val eff' (\v, res => k v (res :: env)) execEff (val :: env) (There p) eff k = execEff env p eff (\v, env' => k v (val :: env')) -- Q: Instead of m b, implement as StateT (Env m xs') m b, so that state -- updates can be propagated even through failing computations? export eff : Env m xs -> EffM m a xs xs' -> ((x : a) -> Env m (xs' x) -> m b) -> m b eff env (Value x) k = k x env eff env (prog `EBind` c) k = eff env prog (\p', env' => eff env' (c p') k) eff env (CallP prf effP) k = execEff env prf effP k eff env (LiftP prf effP) k = let env' = dropEnv env prf in eff env' effP (\p', envk => k p' (rebuildEnv envk prf env)) eff env (New (MkEff resTy newEff) res {prf=Refl} effP) k = eff (res :: env) effP (\p', (val :: envk) => k p' envk) eff env (l :- prog) k = let env' = unlabel env in eff env' prog (\p', envk => k p' (relabel l envk)) -- yuck :) Haven't got interface implementations working nicely in tactic -- proofs yet, and 'search' can't be told about any hints yet, -- so just brute force it. syntax MkDefaultEnv = with Env (| [], [default], [default, default], [default, default, default], [default, default, default, default], [default, default, default, default, default], [default, default, default, default, default, default], [default, default, default, default, default, default, default], [default, default, default, default, default, default, default, default] |) %no_implicit call : {a, b: _} -> {e : Effect} -> (eff : e t a b) -> {auto prf : with List Elem (MkEff a e) xs} -> EffM m t xs (\v => updateResTy v xs prf eff) call e {prf} = CallP prf e implicit lift : EffM m t ys ys' -> {auto prf : SubList ys xs} -> EffM m t xs (\v => updateWith (ys' v) xs prf) lift e {prf} = LiftP prf e -- --------------------------------------------------------- [ Running Effects ] ||| Run an effectful program. ||| ||| The content (`m`) in which to run the program is taken from the ||| environment in which the program is called. The `env` argument is ||| implicit and initialised automatically. ||| ||| @prog The effectful program to run. %no_implicit run : Applicative m => (prog : EffM m a xs xs') -> {default MkDefaultEnv env : Env m xs} -> m a run prog {env} = eff env prog (\r, env => pure r) ||| Run an effectful program in the identity context. ||| ||| A helper function useful for when the given context is 'pure'. ||| The `env` argument is implicit and initialised automatically. ||| ||| @prog The effectful program to run. %no_implicit runPure : (prog : EffM Basics.id a xs xs') -> {default MkDefaultEnv env : Env Basics.id xs} -> a runPure prog {env} = eff env prog (\r, env => r) ||| Run an effectful program in a given context `m` with a default value for the environment. ||| ||| This is useful for when there is no default environment for the given context. ||| ||| @env The environment to use. ||| @prog The effectful program to run. %no_implicit runInit : Applicative m => (env : Env m xs) -> (prog : EffM m a xs xs') -> m a runInit env prog = eff env prog (\r, env => pure r) ||| Run an effectful program with a given default value for the environment. ||| ||| A helper function useful for when the given context is 'pure' and there is no default environment. ||| ||| @env The environment to use. ||| @prog The effectful program to run. %no_implicit runPureInit : (env : Env Basics.id xs) -> (prog : EffM Basics.id a xs xs') -> a runPureInit env prog = eff env prog (\r, env => r) %no_implicit runWith : (a -> m a) -> Env m xs -> EffM m a xs xs' -> m a runWith inj env prog = eff env prog (\r, env => inj r) %no_implicit runEnv : Applicative m => Env m xs -> EffM m a xs xs' -> m (x : a ** Env m (xs' x)) runEnv env prog = eff env prog (\r, env => pure (r ** env)) -- ----------------------------------------------- [ some higher order things ] mapE : (a -> EffM m b xs (\_ => xs)) -> List a -> EffM m (List b) xs (\_ => xs) mapE f [] = pure [] mapE f (x :: xs) = [| f x :: mapE f xs |] mapVE : (a -> EffM m b xs (\_ => xs)) -> Vect n a -> EffM m (Vect n b) xs (\_ => xs) mapVE f [] = pure [] mapVE f (x :: xs) = [| f x :: mapVE f xs |] when : Bool -> Lazy (EffM m () xs (\_ => xs)) -> EffM m () xs (\_ => xs) when True e = Force e when False e = pure () -- --------------------------------------------------------------------- [ EOF ]