feat: case splitting in grind (#6717)

This PR introduces a new feature that allows users to specify which
inductive datatypes the `grind` tactic should perform case splits on.
The configuration option `splitIndPred` is now set to `false` by
default. The attribute `[grind cases]` is used to mark inductive
datatypes and predicates that `grind` may case split on during the
search. Additionally, the attribute `[grind cases eager]` can be used to
mark datatypes and predicates for case splitting both during
pre-processing and the search.

Users can also write `grind [HasType]` or `grind [cases HasType]` to
instruct `grind` to perform case splitting on the inductive predicate
`HasType` in a specific instance. Similarly, `grind [-Or]` can be used
to instruct `grind` not to case split on disjunctions.

Co-authored-by: Leonardo de Moura <leodemoura@amazon.com>

src/Init/Grind/Cases.lean

@@ -5,11 +5,7 @@ Authors: Leonardo de Moura
 -/
 prelude
 import Init.Core
+import Init.Grind.Tactics
 
-attribute [grind_cases] And Prod False Empty True Unit Exists
-
-namespace Lean.Grind.Eager
-
-attribute [scoped grind_cases] Or
-
-end Lean.Grind.Eager
+attribute [grind cases eager] And Prod False Empty True Unit Exists
+attribute [grind cases] Or

src/Init/Grind/Tactics.lean

@@ -48,8 +48,8 @@ structure Config where
   splitIte : Bool := true
   /--
   If `splitIndPred` is `true`, `grind` performs case-splitting on inductive predicates.
-  Otherwise, it performs case-splitting only on types marked with `[grind_split]` attribute. -/
-  splitIndPred : Bool := true
+  Otherwise, it performs case-splitting only on types marked with `[grind cases]` attribute. -/
+  splitIndPred : Bool := false
   /-- By default, `grind` halts as soon as it encounters a sub-goal where no further progress can be made. -/
   failures : Nat := 1
   /-- Maximum number of heartbeats (in thousands) the canonicalizer can spend per definitional equality test. -/

src/Lean/Meta/Tactic/Grind/Attr.lean

@@ -8,57 +8,6 @@ import Lean.Meta.Tactic.Grind.EMatchTheorem
 import Lean.Meta.Tactic.Grind.Cases
 
 namespace Lean.Meta.Grind
---- TODO: delete
-builtin_initialize grindCasesExt : SimpleScopedEnvExtension Name NameSet ←
-  registerSimpleScopedEnvExtension {
-    initial        := {}
-    addEntry       := fun s declName => s.insert declName
-  }
-
-/--
-Returns `true` if `declName` has been tagged with attribute `[grind_cases]`.
--/
-def isGrindCasesTarget (declName : Name) : CoreM Bool :=
-  return grindCasesExt.getState (← getEnv) |>.contains declName
-
-private def getAlias? (value : Expr) : MetaM (Option Name) :=
-  lambdaTelescope value fun _ body => do
-    if let .const declName _ := body.getAppFn' then
-      return some declName
-    else
-      return none
-
-/--
-Throws an error if `declName` cannot be annotated with attribute `[grind_cases]`.
-We support the following cases:
-- `declName` is a non-recursive datatype.
-- `declName` is an abbreviation for a non-recursive datatype.
--/
-private partial def validateGrindCasesAttr (declName : Name) : CoreM Unit := do
-  match (← getConstInfo declName) with
-  | .inductInfo info =>
-    if info.isRec then
-      throwError "`{declName}` is a recursive datatype"
-  | .defnInfo info =>
-    let failed := throwError "`{declName}` is a definition, but it is not an alias/abbreviation for an inductive datatype"
-    let some declName ← getAlias? info.value |>.run' {} {}
-      | failed
-    try
-      validateGrindCasesAttr declName
-    catch _ =>
-      failed
-  | _ =>
-    throwError "`{declName}` is not an inductive datatype or an alias for one"
-
-builtin_initialize
-  registerBuiltinAttribute {
-    name  := `grind_cases
-    descr := "`grind` tactic applies `cases` to (non-recursive) inductives during pre-processing step"
-    add   := fun declName _ attrKind => do
-      validateGrindCasesAttr declName
-      grindCasesExt.add declName attrKind
-  }
---- END of TODO: detele
 
 inductive AttrKind where
   | ematch (k : TheoremKind)

src/Lean/Meta/Tactic/Grind/Cases.lean

@@ -32,6 +32,12 @@ def CasesTypes.insert (s : CasesTypes) (declName : Name) (eager : Bool) : CasesT
 def CasesTypes.find? (s : CasesTypes) (declName : Name) : Option Bool :=
   s.casesMap.find? declName
 
+def CasesTypes.isEagerSplit (s : CasesTypes) (declName : Name) : Bool :=
+  s.casesMap.find? declName |>.getD false
+
+def CasesTypes.isSplit (s : CasesTypes) (declName : Name) : Bool :=
+  s.casesMap.find? declName |>.isSome
+
 builtin_initialize casesExt : SimpleScopedEnvExtension CasesEntry CasesTypes ←
   registerSimpleScopedEnvExtension {
     initial        := {}

src/Lean/Meta/Tactic/Grind/Internalize.lean

@@ -53,7 +53,6 @@ private def addSplitCandidate (e : Expr) : GoalM Unit := do
   trace_goal[grind.split.candidate] "{e}"
   modify fun s => { s with splitCandidates := e :: s.splitCandidates }
 
--- TODO: add attribute to make this extensible
 private def forbiddenSplitTypes := [``Eq, ``HEq, ``True, ``False]
 
 /-- Returns `true` if `e` is of the form `@Eq Prop a b` -/
@@ -63,29 +62,37 @@ def isMorallyIff (e : Expr) : Bool :=
 
 /-- Inserts `e` into the list of case-split candidates if applicable. -/
 private def checkAndAddSplitCandidate (e : Expr) : GoalM Unit := do
-  unless e.isApp do return ()
-  if (← getConfig).splitIte && (e.isIte || e.isDIte) then
-    addSplitCandidate e
-    return ()
-  if isMorallyIff e then
-    addSplitCandidate e
-    return ()
-  if (← getConfig).splitMatch then
-    if (← isMatcherApp e) then
-      if let .reduced _ ← reduceMatcher? e then
-        -- When instantiating `match`-equations, we add `match`-applications that can be reduced,
-        -- and consequently don't need to be splitted.
+  match e with
+  | .app .. =>
+    if (← getConfig).splitIte && (e.isIte || e.isDIte) then
+      addSplitCandidate e
+      return ()
+    if isMorallyIff e then
+      addSplitCandidate e
+      return ()
+    if (← getConfig).splitMatch then
+      if (← isMatcherApp e) then
+        if let .reduced _ ← reduceMatcher? e then
+          -- When instantiating `match`-equations, we add `match`-applications that can be reduced,
+          -- and consequently don't need to be splitted.
+          return ()
+        else
+          addSplitCandidate e
+          return ()
+    let .const declName _  := e.getAppFn | return ()
+      if forbiddenSplitTypes.contains declName then
         return ()
-      else
-        addSplitCandidate e
+      unless (← isInductivePredicate declName) do
         return ()
-  let .const declName _  := e.getAppFn | return ()
-    if forbiddenSplitTypes.contains declName then return ()
-    -- We should have a mechanism for letting users to select types to case-split.
-    -- Right now, we just consider inductive predicates that are not in the forbidden list
-    if (← getConfig).splitIndPred then
-      if (← isInductivePredicate declName) then
+      if (← get).casesTypes.isSplit declName then
         addSplitCandidate e
+      else if (← getConfig).splitIndPred then
+        addSplitCandidate e
+  | .fvar .. =>
+    let .const declName _ := (← inferType e).getAppFn | return ()
+    if (← get).casesTypes.isSplit declName then
+      addSplitCandidate e
+  | _ => pure ()
 
 /--
 If `e` is a `cast`-like term (e.g., `cast h a`), add `HEq e a` to the to-do list.

src/Lean/Meta/Tactic/Grind/Intro.lean

@@ -74,12 +74,12 @@ private def introNext (goal : Goal) (generation : Nat) : GrindM IntroResult := d
   else
     return .done
 
-private def isCasesCandidate (type : Expr) : MetaM Bool := do
+def isEagerCasesCandidate (goal : Goal) (type : Expr) : Bool := Id.run do
   let .const declName _ := type.getAppFn | return false
-  isGrindCasesTarget declName
+  return goal.casesTypes.isEagerSplit declName
 
 private def applyCases? (goal : Goal) (fvarId : FVarId) : MetaM (Option (List Goal)) := goal.mvarId.withContext do
-  if (← isCasesCandidate (← fvarId.getType)) then
+  if isEagerCasesCandidate goal (← fvarId.getType) then
     let mvarIds ← cases goal.mvarId (mkFVar fvarId)
     return mvarIds.map fun mvarId => { goal with mvarId }
   else
@@ -121,7 +121,7 @@ partial def intros  (generation : Nat) : GrindTactic' := fun goal => do
 
 /-- Asserts a new fact `prop` with proof `proof` to the given `goal`. -/
 def assertAt (proof : Expr) (prop : Expr) (generation : Nat) : GrindTactic' := fun goal => do
-  if (← isCasesCandidate prop) then
+  if isEagerCasesCandidate goal prop then
     let mvarId ← goal.mvarId.assert (← mkFreshUserName `h) prop proof
     let goal := { goal with mvarId }
     intros generation goal

src/Lean/Meta/Tactic/Grind/Main.lean

@@ -67,7 +67,8 @@ private def mkGoal (mvarId : MVarId) (params : Params) : GrindM Goal := do
   let falseExpr ← getFalseExpr
   let natZeroExpr ← getNatZeroExpr
   let thmMap := params.ematch
-  GoalM.run' { mvarId, thmMap } do
+  let casesTypes := params.casesTypes
+  GoalM.run' { mvarId, thmMap, casesTypes } do
     mkENodeCore falseExpr (interpreted := true) (ctor := false) (generation := 0)
     mkENodeCore trueExpr (interpreted := true) (ctor := false) (generation := 0)
     mkENodeCore natZeroExpr (interpreted := true) (ctor := false) (generation := 0)

src/Lean/Meta/Tactic/Grind/Split.lean

@@ -101,6 +101,13 @@ private def checkCaseSplitStatus (e : Expr) : GoalM CaseSplitStatus := do
     if let some info ← isInductivePredicate? declName then
       if (← isEqTrue e) then
         return .ready info.ctors.length info.isRec
+    if e.isFVar then
+      let type ← whnfD (← inferType e)
+      let report : GoalM Unit := do
+        reportIssue "cannot perform case-split on {e}, unexpected type{indentExpr type}"
+      let .const declName _ := type.getAppFn | report; return .resolved
+      let .inductInfo info ← getConstInfo declName | report; return .resolved
+      return .ready info.ctors.length info.isRec
     return .notReady
 
 private inductive SplitCandidate where

src/Lean/Meta/Tactic/Grind/Types.lean

@@ -16,6 +16,7 @@ import Lean.Meta.Tactic.Util
 import Lean.Meta.Tactic.Ext
 import Lean.Meta.Tactic.Grind.ENodeKey
 import Lean.Meta.Tactic.Grind.Attr
+import Lean.Meta.Tactic.Grind.Cases
 import Lean.Meta.Tactic.Grind.Arith.Types
 import Lean.Meta.Tactic.Grind.EMatchTheorem
 
@@ -362,6 +363,8 @@ structure Goal where
   nextIdx      : Nat := 0
   /-- State of arithmetic procedures -/
   arith        : Arith.State := {}
+  /-- Inductive datatypes marked for case-splitting -/
+  casesTypes : CasesTypes := {}
   /-- Active theorems that we have performed ematching at least once. -/
   thms         : PArray EMatchTheorem := {}
   /-- Active theorems that we have not performed any round of ematching yet. -/

tests/lean/run/grind_cases.lean

@@ -1,35 +1,3 @@
-/--
-error: `List` is a recursive datatype
--/
-#guard_msgs in
-attribute [grind_cases] List
-
-/--
-error: `Prod.mk` is not an inductive datatype or an alias for one
--/
-#guard_msgs in
-attribute [grind_cases] Prod.mk
-
-/--
-error: `List.append` is a definition, but it is not an alias/abbreviation for an inductive datatype
--/
-#guard_msgs in
-attribute [grind_cases] List.append
-
-attribute [grind_cases] Prod
-
-def Foo (α : Type u) := Sum α α
-
-attribute [grind_cases] Foo
-
-attribute [grind_cases] And
-
-attribute [grind_cases] False
-
-attribute [grind_cases] Empty
-
--- TODO: delete everything above
-
 /--
 error: invalid `[grind cases eager]`, `List` is not a non-recursive inductive datatype or an alias for one
 -/

tests/lean/run/grind_pre.lean

@@ -39,7 +39,9 @@ h : c = true
 theorem ex (h : (f a && (b || f (f c))) = true) (h' : p ∧ q) : b && a := by
   grind
 
-open Lean.Grind.Eager in
+section
+attribute [local grind cases eager] Or
+
 /--
 error: `grind` failed
 case grind.2.1
@@ -69,6 +71,8 @@ h : b = false
 theorem ex2 (h : (f a && (b || f (f c))) = true) (h' : p ∧ q) : b && a := by
   grind
 
+end
+
 def g (i : Nat) (j : Nat) (_ : i > j := by omega) := i + j
 
 /--

tests/lean/run/grind_split.lean

@@ -54,10 +54,13 @@ inductive HasType : Expr → Ty → Prop
 
 set_option trace.grind true
 theorem HasType.det (h₁ : HasType e t₁) (h₂ : HasType e t₂) : t₁ = t₂ := by
-  grind
+  grind [HasType]
 
-theorem HasType.det' (h₁ : HasType e t₁) (h₂ : HasType e t₂) : t₁ = t₂ := by
-  fail_if_success grind (splitIndPred := false)
+example (h₁ : HasType e t₁) (h₂ : HasType e t₂) : t₁ = t₂ := by
+  grind +splitIndPred
+
+example (h₁ : HasType e t₁) (h₂ : HasType e t₂) : t₁ = t₂ := by
+  fail_if_success grind
   sorry
 
 end grind_test_induct_pred

tests/lean/run/grind_split_issue.lean

@@ -47,4 +47,4 @@ h : HEq ⋯ ⋯
 -/
 #guard_msgs (error) in
 example {c : Nat} (q : X c 0) : False := by
-  grind
+  grind [X]

tests/lean/run/grind_t1.lean

@@ -296,7 +296,15 @@ example {α β} (f : α → β) (a : α) : ∃ a', f a' = f a := by
 
 open List in
 example : (replicate n a).map f = replicate n (f a) := by
-  grind only [Option.map_some', Option.map_none', getElem?_map, getElem?_replicate]
+  grind +splitIndPred only [Option.map_some', Option.map_none', getElem?_map, getElem?_replicate]
+
+open List in
+example : (replicate n a).map f = replicate n (f a) := by
+  grind only [Exists, Option.map_some', Option.map_none', getElem?_map, getElem?_replicate]
+
+open List in
+example : (replicate n a).map f = replicate n (f a) := by
+  grind only [cases Exists, Option.map_some', Option.map_none', getElem?_map, getElem?_replicate]
 
 open List in
 example : (replicate n a).map f = replicate n (f a) := by