feat: Refactor out projToApp

Pantograph/Expr.lean

@@ -4,6 +4,26 @@ open Lean
 
 namespace Pantograph
 
+structure ProjectionApplication where
+  projector: Name
+  numParams: Nat
+  inner: Expr
+
+@[export pantograph_expr_proj_to_app]
+def exprProjToApp (env: Environment) (e: Expr): ProjectionApplication :=
+  let (typeName, idx, inner) := match e with
+    | .proj typeName idx inner => (typeName, idx, inner)
+    | _ => panic! "Argument must be proj"
+  let ctor := getStructureCtor env typeName
+  let fieldName := getStructureFields env typeName |>.get! idx
+  let projector := getProjFnForField? env typeName fieldName |>.get!
+  {
+    projector,
+    numParams := ctor.numParams,
+    inner,
+  }
+
+
 def _root_.Lean.Name.isAuxLemma (n : Lean.Name) : Bool := n matches .num (.str _ "_auxLemma") _
 
 /-- Unfold all lemmas created by `Lean.Meta.mkAuxLemma`. These end in `_auxLemma.nn` where `nn` is a number. -/

Pantograph/Serial.lean

@@ -168,15 +168,12 @@ partial def serializeExpressionSexp (expr: Expr) (sanitize: Bool := true): MetaM
       -- NOTE: Equivalent to expr itself, but mdata influences the prettyprinter
       -- It may become necessary to incorporate the metadata.
       self inner
-    | .proj typeName idx inner => do
+    | .proj _ _ _ => do
       let env ← getEnv
-      let ctor := getStructureCtor env typeName
-      let fieldName := getStructureFields env typeName |>.get! idx
-      let projectorName := getProjFnForField? env typeName fieldName |>.get!
-
-      let autos := String.intercalate " " (List.replicate ctor.numParams "_")
-      let inner ← self inner
-      pure s!"((:c {projectorName}) {autos} {inner})"
+      let projApp := exprProjToApp env e
+      let autos := String.intercalate " " (List.replicate projApp.numParams "_")
+      let inner ← self projApp.inner
+      pure s!"((:c {projApp.projector}) {autos} {inner})"
   -- Elides all unhygenic names
   binderInfoSexp : Lean.BinderInfo → String
     | .default => ""