[refactor] vectorise: take expected mode, make return type explicit (#58

)

It's confusing where responsibility for error handling (expected
function, got wrong mode/type) belongs, hence "-- These shouldn't
happen" type comments - so, make that clear, removing those comments :)

This shortens quite a lot of stuff and makes `getThunks` much simpler,
 albeit at the cost of a nasty case/match in `mkMapFun` because
`next` loses the type of what we've passed in. (A problem in common with
other clients of `next`.)
---------

Co-authored-by: Craig Roy <[email protected]>

brat/Brat/Checker/Helpers.hs

@@ -38,6 +38,7 @@ import Util (log2)
 
 import Control.Monad.Freer (req)
 import Data.Bifunctor
+import Data.Foldable (foldrM)
 import Data.Type.Equality (TestEquality(..), (:~:)(..))
 import qualified Data.Map as M
 import Prelude hiding (last)
@@ -247,22 +248,19 @@ getThunks :: Modey m
                       ,Overs m UVerb
                       )
 getThunks _ [] = pure ([], [], [])
-getThunks Braty row@((src, Right ty):rest) = (eval S0 ty >>= vectorise . (src,)) >>= \case
-  (src, VFun Braty (ss :->> ts)) -> do
-    (node, unders, overs, _) <- let ?my = Braty in
-                                  anext "" (Eval (end src)) (S0, Some (Zy :* S0)) ss ts
-    (nodes, unders', overs') <- getThunks Braty rest
-    pure (node:nodes, unders <> unders', overs <> overs')
-  -- These shouldn't happen
-  (_, VFun _ _) -> err $ ExpectedThunk (showMode Braty) (showRow row)
-  v -> typeErr $ "Force called on non-thunk: " ++ show v
-getThunks Kerny row@((src, Right ty):rest) = (eval S0 ty >>= vectorise . (src,)) >>= \case
-  (src, VFun Kerny (ss :->> ts)) -> do
-    (node, unders, overs, _) <- let ?my = Kerny in anext "" (Splice (end src)) (S0, Some (Zy :* S0)) ss ts
-    (nodes, unders', overs') <- getThunks Kerny rest
-    pure (node:nodes, unders <> unders', overs <> overs')
-  (_, VFun _ _) -> err $ ExpectedThunk (showMode Kerny) (showRow row)
-  v -> typeErr $ "Force called on non-(kernel)-thunk: " ++ show v
+getThunks Braty ((src, Right ty):rest) = do
+  ty <- eval S0 ty
+  (src, (ss :->> ts)) <- vectorise Braty (src, ty)
+  (node, unders, overs, _) <- let ?my = Braty in
+                                anext "" (Eval (end src)) (S0, Some (Zy :* S0)) ss ts
+  (nodes, unders', overs') <- getThunks Braty rest
+  pure (node:nodes, unders <> unders', overs <> overs')
+getThunks Kerny ((src, Right ty):rest) = do
+  ty <- eval S0 ty
+  (src, (ss :->> ts)) <- vectorise Kerny (src,ty)
+  (node, unders, overs, _) <- let ?my = Kerny in anext "" (Splice (end src)) (S0, Some (Zy :* S0)) ss ts
+  (nodes, unders', overs') <- getThunks Kerny rest
+  pure (node:nodes, unders <> unders', overs <> overs')
 getThunks Braty ((src, Left (Star args)):rest) = do
   (node, unders, overs) <- case bwdStack (B0 <>< args) of
     Some (_ :* stk) -> do
@@ -274,15 +272,15 @@ getThunks Braty ((src, Left (Star args)):rest) = do
 getThunks m ro = err $ ExpectedThunk (showMode m) (showRow ro)
 
 -- The type given here should be normalised
-vecLayers :: Val Z -> Checking ([(Src, NumVal (VVar Z))] -- The sizes of the vector layers
-                               ,Some (Modey :* Flip CTy Z) -- The function type at the end
-                               )
-vecLayers (TVec ty (VNum n)) = do
+vecLayers :: Modey m -> Val Z -> Checking ([(Src, NumVal (VVar Z))] -- The sizes of the vector layers
+                                          ,CTy m Z -- The function type at the end
+                                          )
+vecLayers my (TVec ty (VNum n)) = do
   src <- mkStaticNum n
-  (layers, fun) <- vecLayers ty
-  pure ((src, n):layers, fun)
-vecLayers (VFun my cty) = pure ([], Some (my :* Flip cty))
-vecLayers ty = typeErr $ "Expected a function or vector of functions, got " ++ show ty
+  first ((src, n):) <$> vecLayers my ty
+vecLayers Braty (VFun Braty cty) = pure ([], cty)
+vecLayers Kerny (VFun Kerny cty) = pure ([], cty)
+vecLayers my ty = typeErr $ "Expected a " ++ showMode my ++ "function or vector of functions, got " ++ show ty
 
 mkStaticNum :: NumVal (VVar Z) -> Checking Src
 mkStaticNum n@(NumValue c gro) = do
@@ -330,27 +328,29 @@ mkStaticNum n@(NumValue c gro) = do
     wire (oneSrc, TNat, rhs)
     pure src
 
-vectorise :: (Src, Val Z) -> Checking (Src, Val Z)
-vectorise (src, ty) = do
-  (layers, Some (my :* Flip cty)) <- vecLayers ty
-  modily my $ mkMapFuns (src, VFun my cty) layers
+vectorise :: forall m. Modey m -> (Src, Val Z) -> Checking (Src, CTy m Z)
+vectorise my (src, ty) = do
+  (layers, cty) <- vecLayers my ty
+  modily my $ foldrM mkMapFun (src, cty) layers
  where
-  mkMapFuns :: (Src, Val Z) -- The input to the mapfun
-            -> [(Src, NumVal (VVar Z))] -- Remaining layers
-            -> Checking (Src, Val Z)
-  mkMapFuns over [] = pure over
-  mkMapFuns (valSrc, ty) ((lenSrc, len):layers) = do
-    (valSrc, ty@(VFun my cty)) <- mkMapFuns (valSrc, ty) layers
+  mkMapFun :: (Src, NumVal (VVar Z)) -- Layer to apply
+            -> (Src, CTy m Z) -- The input to this level of mapfun
+            -> Checking (Src, CTy m Z)
+  mkMapFun (lenSrc, len) (valSrc, cty) = do
     let weak1 = changeVar (Thinning (ThDrop ThNull))
     vecFun <- vectorisedFun len my cty
-    (_, [(lenTgt,_), (valTgt, _)], [(vectorSrc, Right vecTy)], _) <-
+    (_, [(lenTgt,_), (valTgt, _)], [(vectorSrc, Right (VFun my' cty))], _) <-
       next "" MapFun (S0, Some (Zy :* S0))
       (REx ("len", Nat) (RPr ("value", weak1 ty) R0))
       (RPr ("vector", weak1 vecFun) R0)
     defineTgt lenTgt (VNum len)
     wire (lenSrc, kindType Nat, lenTgt)
     wire (valSrc, ty, valTgt)
-    pure (vectorSrc, vecTy)
+    let vecCTy = case (my,my',cty) of
+          (Braty,Braty,cty) -> cty
+          (Kerny,Kerny,cty) -> cty
+          _ -> error "next returned wrong mode of computation type to that passed in"
+    pure (vectorSrc, vecCTy)
 
   vectorisedFun :: NumVal (VVar Z) -> Modey m -> CTy m Z -> Checking (Val Z)
   vectorisedFun nv my (ss :->> ts) = do

brat/test/golden/kernel/kernel_application.brat.golden

@@ -2,7 +2,5 @@ Error in test/golden/kernel/kernel_application.brat on line 16:
 rotate = { q => maybeRotate(true) }
                 ^^^^^^^^^^^
 
-  Expected function to be a (kernel) thunk, but found:
-  (thunk :: { (a1 :: Bool) -> (a1 :: { (a1 :: Qubit) -o (a1 :: Qubit) }) })
-
+  Type error: Expected a (kernel) function or vector of functions, got { (a1 :: Bool) -> (a1 :: { (a1 :: Qubit) -o (a1 :: Qubit) }) }