feat: store outParam positions

src/Lean/Class.lean

@@ -8,8 +8,16 @@ import Lean.Attributes
 namespace Lean
 
 structure ClassEntry where
-  name : Name
-  hasOutParam : Bool
+  name      : Name
+  /--
+    Position of the class `outParams`.
+    For example, for class
+    ```
+    class GetElem (Cont : Type u) (Idx : Type v) (Elem : outParam (Type w)) (Dom : outParam (Cont → Idx → Prop)) where
+    ```
+    `outParams := #[2, 3]`
+  -/
+  outParams : Array Nat
 
 namespace ClassEntry
 
@@ -19,16 +27,16 @@ def lt (a b : ClassEntry) : Bool :=
 end ClassEntry
 
 structure ClassState where
-  hasOutParam : SMap Name Bool := SMap.empty
+  outParamMap : SMap Name (Array Nat) := SMap.empty
   deriving Inhabited
 
 namespace ClassState
 
 def addEntry (s : ClassState) (entry : ClassEntry) : ClassState :=
-  { s with hasOutParam := s.hasOutParam.insert entry.name entry.hasOutParam }
+  { s with outParamMap := s.outParamMap.insert entry.name entry.outParams }
 
 def switch (s : ClassState) : ClassState :=
-  { s with hasOutParam := s.hasOutParam.switch }
+  { s with outParamMap := s.outParamMap.switch }
 
 end ClassState
 
@@ -42,17 +50,23 @@ builtin_initialize classExtension : SimplePersistentEnvExtension ClassEntry Clas
 
 @[export lean_is_class]
 def isClass (env : Environment) (n : Name) : Bool :=
-  (classExtension.getState env).hasOutParam.contains n
-
-@[export lean_has_out_params]
-def hasOutParams (env : Environment) (n : Name) : Bool :=
-  match (classExtension.getState env).hasOutParam.find? n with
-  | some b => b
-  | none   => false
+  (classExtension.getState env).outParamMap.contains n
 
 /--
-  Auxiliary function for checking whether a class has `outParam`, and
-  whether they are being correctly used.
+  If `declName` is a class, return the position of its `outParams`.
+-/
+def getOutParamPositions? (env : Environment) (declName : Name) : Option (Array Nat) :=
+  (classExtension.getState env).outParamMap.find? declName
+
+@[export lean_has_out_params]
+def hasOutParams (env : Environment) (declName : Name) : Bool :=
+  match getOutParamPositions? env declName with
+  | some outParams => !outParams.isEmpty
+  | none => false
+
+/--
+  Auxiliary function for collection the position class `outParams`, and
+  checking whether they are being correctly used.
   A regular (i.e., non `outParam`) must not depend on an `outParam`.
   Reason for this restriction:
   When performing type class resolution, we replace arguments that
@@ -62,19 +76,19 @@ def hasOutParams (env : Environment) (n : Name) : Bool :=
   incorrect. This transformation would be counterintuitive to users since
   we would implicitly treat these regular parameters as `outParam`s.
 -/
-private partial def checkOutParam : Nat → Array FVarId → Expr → Except String Bool
-  | i, outParams, Expr.forallE _ d b _ =>
+private partial def checkOutParam (i : Nat) (outParamFVarIds : Array FVarId) (outParams : Array Nat) (type : Expr) : Except String (Array Nat) :=
+  match type with
+  | .forallE _ d b _ =>
     if d.isOutParam then
-      let fvarId := { name := Name.mkNum `_fvar outParams.size }
-      let outParams := outParams.push fvarId
+      let fvarId := { name := Name.mkNum `_fvar outParamFVarIds.size }
       let fvar      := mkFVar fvarId
       let b         := b.instantiate1 fvar
-      checkOutParam (i+1) outParams b
-    else if d.hasAnyFVar fun fvarId => outParams.contains fvarId then
-      Except.error s!"invalid class, parameter #{i} depends on `outParam`, but it is not an `outParam`"
+      checkOutParam (i+1) (outParamFVarIds.push fvarId) (outParams.push i) b
+    else if d.hasAnyFVar fun fvarId => outParamFVarIds.contains fvarId then
+      Except.error s!"invalid class, parameter #{i+1} depends on `outParam`, but it is not an `outParam`"
     else
-      checkOutParam (i+1) outParams b
-  | _, outParams, _ => pure (outParams.size > 0)
+      checkOutParam (i+1) outParamFVarIds outParams b
+  | _ => return outParams
 
 def addClass (env : Environment) (clsName : Name) : Except String Environment := do
   if isClass env clsName then
@@ -83,17 +97,17 @@ def addClass (env : Environment) (clsName : Name) : Except String Environment :=
     | throw s!"unknown declaration '{clsName}'"
   unless decl matches .inductInfo .. | .axiomInfo .. do
     throw s!"invalid 'class', declaration '{clsName}' must be inductive datatype, structure, or constant"
-  let b ← checkOutParam 1 #[] decl.type
-  return classExtension.addEntry env { name := clsName, hasOutParam := b }
+  let outParams ← checkOutParam 0 #[] #[] decl.type
+  return classExtension.addEntry env { name := clsName, outParams }
 
 private def consumeNLambdas : Nat → Expr → Option Expr
-  | 0,   e                => some e
-  | i+1, Expr.lam _ _ b _ => consumeNLambdas i b
-  | _,   _                => none
+  | 0,   e            => some e
+  | i+1, .lam _ _ b _ => consumeNLambdas i b
+  | _,   _            => none
 
 partial def getClassName (env : Environment) : Expr → Option Name
-  | Expr.forallE _ _ b _ => getClassName env b
-  | e                    => do
+  | .forallE _ _ b _ => getClassName env b
+  | e                => do
     let Expr.const c _ ← pure e.getAppFn | none
     let info ← env.find? c
     match info.value? with
@@ -106,8 +120,8 @@ partial def getClassName (env : Environment) : Expr → Option Name
 
 builtin_initialize
   registerBuiltinAttribute {
-    name  := `class,
-    descr := "type class",
+    name  := `class
+    descr := "type class"
     add   := fun decl stx kind => do
       let env ← getEnv
       Attribute.Builtin.ensureNoArgs stx