feat: support for arrow types in the dot notation

cc @gebner

RELEASES.md

@@ -1,6 +1,9 @@
 v4.0.0-m4 (WIP)
 ---------
 
+* Extend dot-notation `x.field` for arrow types. If type of `x` is an arrow, we look up for `Function.field`.
+For example, given `f : Nat → Nat` and `g : Nat → Nat`, `f.comp g` is now notation for `Function.comp f g`.
+
 * [Add code folding support to the language server](https://github.com/leanprover/lean4/pull/1014).
 
 * Support notation `let <pattern> := <expr> | <else-case>` in `do` blocks.

src/Lean/Elab/App.lean

@@ -538,7 +538,17 @@ private partial def findMethod? (env : Environment) (structName fieldName : Name
       else
         none
 
+private def throwInvalidFieldNotation (e eType : Expr) : TermElabM α :=
+  throwLValError e eType "invalid field notation, type is not of the form (C ...) where C is a constant"
+
 private def resolveLValAux (e : Expr) (eType : Expr) (lval : LVal) : TermElabM LValResolution := do
+  if eType.isForall then
+    match lval with
+    | LVal.fieldName _ fieldName _ _ =>
+      let fullName := `Function ++ fieldName
+      if (← getEnv).contains fullName then
+        return LValResolution.const `Function `Function fullName
+    | _ => pure ()
   match eType.getAppFn.constName?, lval with
   | some structName, LVal.fieldIdx _ idx =>
     if idx == 0 then
@@ -595,11 +605,9 @@ private def resolveLValAux (e : Expr) (eType : Expr) (lval : LVal) : TermElabM L
     if e.isConst then
       throwUnknownConstant (e.constName! ++ suffix)
     else
-      throwLValError e eType "invalid field notation, type is not of the form (C ...) where C is a constant"
-  | _, LVal.getOp _ idx =>
-    throwLValError e eType "invalid [..] notation, type is not of the form (C ...) where C is a constant"
-  | _, _ =>
-    throwLValError e eType "invalid field notation, type is not of the form (C ...) where C is a constant"
+      throwInvalidFieldNotation e eType
+  | _, LVal.getOp _ idx => throwInvalidFieldNotation e eType
+  | _, _ => throwInvalidFieldNotation e eType
 
 /- whnfCore + implicit consumption.
    Example: given `e` with `eType := {α : Type} → (fun β => List β) α `, it produces `(e ?m, List ?m)` where `?m` is fresh metavariable. -/
@@ -653,6 +661,14 @@ private partial def mkBaseProjections (baseStructName : Name) (structName : Name
       e ← elabAppArgs projFn #[{ name := `self, val := Arg.expr e }] (args := #[]) (expectedType? := none) (explicit := false) (ellipsis := false)
     return e
 
+private def typeMatchesBaseName (type : Expr) (baseName : Name) : MetaM Bool := do
+  if baseName == `Function then
+    return (← whnfR type).isForall
+  else if type.consumeMData.isAppOf baseName then
+    return true
+  else
+    return (← whnfR type).isAppOf baseName
+
 /- Auxiliary method for field notation. It tries to add `e` as a new argument to `args` or `namedArgs`.
    This method first finds the parameter with a type of the form `(baseName ...)`.
    When the parameter is found, if it an explicit one and `args` is big enough, we add `e` to `args`.
@@ -672,16 +688,8 @@ private def addLValArg (baseName : Name) (fullName : Name) (e : Expr) (args : Ar
       | some idx =>
         remainingNamedArgs := remainingNamedArgs.eraseIdx idx
       | none =>
-        let mut foundIt := false
         let type := xDecl.type
-        if type.consumeMData.isAppOf baseName then
-          foundIt := true
-        if !foundIt then
-          /- Normalize type and try again -/
-          let type ← withReducible $ whnf type
-          if type.consumeMData.isAppOf baseName then
-            foundIt := true
-        if foundIt then
+        if (← typeMatchesBaseName type baseName) then
           /- We found a type of the form (baseName ...).
              First, we check if the current argument is an explicit one,
              and the current explicit position "fits" at `args` (i.e., it must be ≤ arg.size) -/

tests/lean/run/arrowDot.lean

@@ -0,0 +1,5 @@
+def test (f : Nat → Nat) (g : Nat → Nat) :=
+  f.comp g $ 10
+
+example : test (·+1) (·*2) = 21 :=
+  rfl