Remove the Either Int from value2term

This prevents HUGE space leak and makes compiling a PGF a LOT faster

For example, an application grammar moved from taking over 50GB
of ram and taking 5 minutes (most of which is spent on garbage colelction)
to taking 1.2 seconds and using 42mb of memory

The price we pay is that the "variable #n is out of scope" error is now
lazy and will happen when we try to evaluate the term instead of
happening when the function returns and allowing the caller to chose how
to handle the error.
I don't think this should matter in practice, since it's very rare;
at least Inari has never encountered it.

src/compiler/GF/Compile/Compute/Concrete.hs

@@ -493,57 +493,59 @@ vtrace loc arg res = trace (render (hang (pv arg) 4 ("->"<+>pv res))) res
     pf (_,v) = ppV v
     pa (_,v) = ppV v
     ppV v = case value2term' True loc [] v of
-              Left i  -> "variable #" <> pp i <+> "is out of scope"
-              Right t -> ppTerm Unqualified 10 t
+              -- Left i  -> "variable #" <> pp i <+> "is out of scope"
+              t -> ppTerm Unqualified 10 t
 
 -- | Convert a value back to a term
 value2term :: GLocation -> [Ident] -> Value -> Either Int Term
-value2term = value2term' False
+value2term loc xs v0 = Right $ value2term' False loc xs v0
+
+value2term' :: Bool -> p -> [Ident] -> Value -> Term
 value2term' stop loc xs v0 =
   case v0 of
-    VApp pre vs    -> liftM (foldl App (Q (cPredef,predefName pre))) (mapM v2t vs)
-    VCApp f vs     -> liftM  (foldl App (QC f))   (mapM v2t vs)
-    VGen j vs      -> liftM2 (foldl App) (var j)  (mapM v2t vs)
-    VMeta j env vs -> liftM  (foldl App (Meta j)) (mapM v2t vs)
-    VProd bt v x f -> liftM2 (Prod bt x) (v2t v) (v2t' x f)
-    VAbs  bt   x f -> liftM  (Abs  bt x)         (v2t' x f)
-    VInt n         -> return (EInt n)
-    VFloat f       -> return (EFloat f)
-    VString s      -> return (if null s then Empty else K s)
-    VSort s        -> return (Sort s)
-    VImplArg v     -> liftM  ImplArg (v2t v)
-    VTblType p res -> liftM2 Table (v2t p) (v2t res)
-    VRecType rs    -> liftM  RecType (mapM (\(l,v) -> fmap ((,) l) (v2t v)) rs)
-    VRec as        -> liftM  R       (mapM (\(l,v) -> v2t v >>= \t -> return (l,(Nothing,t))) as)
-    VV t _ vs      -> liftM  (V t)   (mapM v2t vs)
-    VT wild v cs   -> v2t v >>= \t -> liftM (T ((if wild then TWild else TTyped) t)) (mapM nfcase cs)
-    VFV vs         -> liftM  FV (mapM v2t vs)
-    VC v1 v2       -> liftM2 C (v2t v1) (v2t v2)
-    VS v1 v2       -> liftM2 S (v2t v1) (v2t v2)
-    VP v l         -> v2t v >>= \t -> return (P t l)
-    VPatt p        -> return (EPatt p)
-    VPattType v    -> v2t v >>= return . EPattType
-    VAlts v vvs    -> liftM2 Alts (v2t v) (mapM (\(x,y) -> liftM2 (,) (v2t x) (v2t y)) vvs)
-    VStrs vs       -> liftM  Strs (mapM v2t vs)
+    VApp pre vs    -> applyMany (Q (cPredef,predefName pre)) vs
+    VCApp f vs     -> applyMany (QC f)                       vs
+    VGen j vs      -> applyMany (var j)                      vs
+    VMeta j env vs -> applyMany (Meta j)                     vs
+    VProd bt v x f -> Prod bt x (v2t v)  (v2t' x f)
+    VAbs  bt   x f -> Abs  bt x          (v2t' x f)
+    VInt n         -> EInt n
+    VFloat f       -> EFloat f
+    VString s      -> if null s then Empty else K s
+    VSort s        -> Sort s
+    VImplArg v     -> ImplArg (v2t v)
+    VTblType p res -> Table (v2t p) (v2t res)
+    VRecType rs    -> RecType [(l, v2t v) | (l,v) <- rs]
+    VRec as        -> R       [(l, (Nothing, v2t v)) | (l,v) <- as]
+    VV t _ vs      -> V t     (map v2t vs)
+    VT wild v cs   -> T ((if wild then TWild else TTyped) (v2t v)) (map nfcase cs)
+    VFV vs         -> FV (map v2t vs)
+    VC v1 v2       -> C (v2t v1) (v2t v2)
+    VS v1 v2       -> S (v2t v1) (v2t v2)
+    VP v l         -> P (v2t v) l
+    VPatt p        -> EPatt p
+    VPattType v    -> EPattType $ v2t v
+    VAlts v vvs    -> Alts (v2t v) [(v2t x, v2t y) | (x,y) <- vvs]
+    VStrs vs       -> Strs (map v2t vs)
 --  VGlue v1 v2    -> Glue (v2t v1) (v2t v2)
 --  VExtR v1 v2    -> ExtR (v2t v1) (v2t v2)
-    VError err     -> return (Error err)
-
+    VError err     -> Error err
   where
+    applyMany f vs = foldl App f (map v2t vs)
     v2t = v2txs xs
     v2txs = value2term' stop loc
     v2t' x f = v2txs (x:xs) (bind f (gen xs))
 
     var j
-      | j<length xs = Right (Vr (reverse xs !! j))
-      | otherwise   = Left  j
+      | j<length xs = Vr (reverse xs !! j)
+      | otherwise   = error ("variable #"++show j++" is out of scope")
 
 
     pushs xs e = foldr push e xs
     push x (env,xs) = ((x,gen xs):env,x:xs)
     gen xs = VGen (length xs) []
 
-    nfcase (p,f) = liftM ((,) p) (v2txs xs' (bind f env'))
+    nfcase (p,f) = (,) p (v2txs xs' (bind f env'))
       where (env',xs') = pushs (pattVars p) ([],xs)
 
     bind (Bind f) x = if stop