Address a comment in PR 5160. (#5211)

* Address a comment in PR 5160.

Co-authored-by: Michael Peyton Jones <[email protected]>

Author: Marty Stumpf

plutus-core/plutus-ir/src/PlutusIR/Transform/Inline/CallSiteInline.hs

@@ -142,23 +142,34 @@ Also, we currently reject `Constant` (has acceptable cost but not acceptable siz
 We may want to check their sizes instead of just rejecting them.
 -}
 
--- | A list of `LamAbs` and `TyAbs`, in order, of a let-binding.
-type TermOrTypeOrder = [TermOrType]
+{-|
+The (syntactic) arity of a term. That is, a record of the arguments that the
+term expects before it may do some work. Since we have both type and lambda
+abstractions, this is not a simple argument count, but rather a list of values
+indicating whether the next argument should be a term or a type.
+
+Note that this is the syntactic arity, i.e. it just corresponds to the number of
+syntactic lambda and type abstractions on the outside of the term. It is thus
+an under-approximation of how many arguments the term may need.
+e.g. consider the term @let id = \x -> x in id@: the variable @id@ has syntactic
+arity @[]@, but does in fact need an argument before it does any work.
+-}
+type Arity = [TermOrType]
 
--- | Datatype capturing both terms and types.
+-- | Is the next argument a term or a type?
 data TermOrType =
     MkTerm | MkType
     deriving stock (Eq, Show)
 
 instance Pretty TermOrType where
   pretty = viaShow
 
--- | Counts the type and term lambdas in the RHS of a binding and returns an ordered list
-countLam ::
-    Term tyname name uni fun a -- ^ the RHS of the let binding
-    -> TermOrTypeOrder
-countLam = \case
-    LamAbs _ _ _ body -> MkTerm : countLam body
-    TyAbs _ _ _ body  -> MkType : countLam body
-    -- Whenever we encounter a body that is not a lambda abstraction, we are done counting
+-- | Computes the 'Arity' of a term.
+computeArity ::
+    Term tyname name uni fun a
+    -> Arity
+computeArity = \case
+    LamAbs _ _ _ body -> MkTerm : computeArity body
+    TyAbs _ _ _ body  -> MkType : computeArity body
+    -- Whenever we encounter a body that is not a lambda or type abstraction, we are done counting
     _                 -> []

plutus-core/plutus-ir/test/TransformSpec.hs

@@ -19,7 +19,7 @@ import PlutusIR.Parser
 import PlutusIR.Test
 import PlutusIR.Transform.Beta qualified as Beta
 import PlutusIR.Transform.DeadCode qualified as DeadCode
-import PlutusIR.Transform.Inline.CallSiteInline (countLam)
+import PlutusIR.Transform.Inline.CallSiteInline (computeArity)
 import PlutusIR.Transform.Inline.UnconditionalInline qualified as UInline
 import PlutusIR.Transform.LetFloatIn qualified as LetFloatIn
 import PlutusIR.Transform.LetFloatOut qualified as LetFloatOut
@@ -42,7 +42,7 @@ transform =
         , letFloatInRelaxed
         , recSplit
         , inline
-        , countLamTest
+        , computeArityTest
         , beta
         , unwrapCancel
         , deadCode
@@ -211,10 +211,10 @@ inline =
             , "letOverAppMulti" -- multiple occurrences of an over-application of a function
             ]
 
-countLamTest :: TestNested
-countLamTest = testNested "countLamTest" $
+computeArityTest :: TestNested
+computeArityTest = testNested "computeArityTest" $
         map
-            (goldenPir (countLam . runQuote . PLC.rename) pTerm)
+            (goldenPir (computeArity . runQuote . PLC.rename) pTerm)
             [ "var" -- from inline tests, testing let terms
             , "tyvar"
             , "single"