lean4-htt/src/Lean/Meta/Closure.lean

/-
Copyright (c) 2020 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Leonardo de Moura
-/
prelude
import Lean.MetavarContext
import Lean.Environment
import Lean.AddDecl
import Lean.Util.FoldConsts
import Lean.Meta.Basic
import Lean.Meta.Check
import Lean.Meta.Tactic.AuxLemma

/-!

This module provides functions for "closing" open terms and
creating auxiliary definitions. Here, we say a term is "open" if
it contains free/meta-variables.

The "closure" is performed by lambda abstracting the
free/meta-variables. Recall that in dependent type theory
lambda abstracting a let-variable may produce type incorrect terms.
For example, given the context
```lean
(n : Nat := 20)
(x : Vector α n)
(y : Vector α 20)
```
the term `x = y` is correct. However, its closure using lambda abstractions
is not.
```lean
fun (n : Nat) (x : Vector α n) (y : Vector α 20) => x = y
```
A previous version of this module would address this issue by
always use let-expressions to abstract let-vars. In the example above,
it would produce
```lean
let n : Nat := 20; fun (x : Vector α n) (y : Vector α 20) => x = y
```
This approach produces correct result, but produces unsatisfactory
results when we want to create auxiliary definitions.
For example, consider the context
```lean
(x : Nat)
(y : Nat := fact x)
```
and the term `h (g y)`, now suppose we want to create an auxiliary definition for `y`.
 The previous version of this module would compute the auxiliary definition
```lean
def aux := fun (x : Nat) => let y : Nat := fact x; h (g y)
```
and would return the term `aux x` as a substitute for `h (g y)`.
This is correct, but we will re-evaluate `fact x` whenever we use `aux`.
In this module, we produce
```lean
def aux := fun (y : Nat) => h (g y)
```
Note that in this particular case, it is safe to lambda abstract the let-varible `y`.
This module uses the following approach to decide whether it is safe or not to lambda
abstract a let-variable.
1) We enable zetaDelta-expansion tracking in `MetaM`. That is, whenever we perform type checking
   if a let-variable needs to zetaDelta expanded, we store it in the set `zetaDeltaFVarIds`.
   We say a let-variable is zetaDelta expanded when we replace it with its value.
2) We use the `MetaM` type checker `check` to type check the expression we want to close,
   and the type of the binders.
3) If a let-variable is not in `zetaDeltaFVarIds`, we lambda abstract it.

Remark: We still use let-expressions for let-variables in `zetaDeltaFVarIds`, but we move the
`let` inside the lambdas. The idea is to make sure the auxiliary definition does not have
an interleaving of `lambda` and `let` expressions. Thus, if the let-variable occurs in
the type of one of the lambdas, we simply zeta-expand it there.
As a final example consider the context
```lean
(x_1 : Nat)
(x_2 : Nat)
(x_3 : Nat)
(x   : Nat := fact (10 + x_1 + x_2 + x_3))
(ty  : Type := Nat → Nat)
(f   : ty := fun x => x)
(n   : Nat := 20)
(z   : f 10)
```
and we use this module to compute an auxiliary definition for the term
```lean
(let y  : { v : Nat // v = n } := ⟨20, rfl⟩; y.1 + n + f x, z + 10)
```
we obtain
```lean
def aux (x : Nat) (f : Nat → Nat) (z : Nat) : Nat×Nat :=
let n : Nat := 20;
(let y : {v // v=n} := {val := 20, property := ex._proof_1}; y.val+n+f x, z+10)
```

BTW, this module also provides the `zetaDelta : Bool` flag. When set to true, it
expands all let-variables occurring in the target expression.
-/

namespace Lean.Meta
namespace Closure

structure ToProcessElement where
  fvarId : FVarId
  newFVarId : FVarId
  deriving Inhabited

structure Context where
  zetaDelta : Bool

structure State where
  visitedLevel          : LevelMap Level := {}
  visitedExpr           : ExprStructMap Expr := {}
  levelParams           : Array Name := #[]
  nextLevelIdx          : Nat := 1
  levelArgs             : Array Level := #[]
  newLocalDecls         : Array LocalDecl := #[]
  newLocalDeclsForMVars : Array LocalDecl := #[]
  newLetDecls           : Array LocalDecl := #[]
  nextExprIdx           : Nat := 1
  exprMVarArgs          : Array Expr := #[]
  exprFVarArgs          : Array Expr := #[]
  toProcess             : Array ToProcessElement := #[]

abbrev ClosureM := ReaderT Context $ StateRefT State MetaM

@[inline] def visitLevel (f : Level → ClosureM Level) (u : Level) : ClosureM Level := do
  if !u.hasMVar && !u.hasParam then
    pure u
  else
    let s ← get
    match s.visitedLevel[u]? with
    | some v => pure v
    | none   => do
      let v ← f u
      modify fun s => { s with visitedLevel := s.visitedLevel.insert u v }
      pure v

@[inline] def visitExpr (f : Expr → ClosureM Expr) (e : Expr) : ClosureM Expr := do
  if !e.hasLevelParam && !e.hasFVar && !e.hasMVar then
    pure e
  else
    let s ← get
    match s.visitedExpr.get? e with
    | some r => pure r
    | none   =>
      let r ← f e
      modify fun s => { s with visitedExpr := s.visitedExpr.insert e r }
      pure r

def mkNewLevelParam (u : Level) : ClosureM Level := do
  let s ← get
  let p := (`u).appendIndexAfter s.nextLevelIdx
  modify fun s => { s with levelParams := s.levelParams.push p, nextLevelIdx := s.nextLevelIdx + 1, levelArgs := s.levelArgs.push u }
  pure $ mkLevelParam p

partial def collectLevelAux : Level → ClosureM Level
  | u@(Level.succ v)   => return u.updateSucc! (← visitLevel collectLevelAux v)
  | u@(Level.max v w)  => return u.updateMax! (← visitLevel collectLevelAux v) (← visitLevel collectLevelAux w)
  | u@(Level.imax v w) => return u.updateIMax! (← visitLevel collectLevelAux v) (← visitLevel collectLevelAux w)
  | u@(Level.mvar ..)    => mkNewLevelParam u
  | u@(Level.param ..)   => mkNewLevelParam u
  | u@(Level.zero)     => pure u

def collectLevel (u : Level) : ClosureM Level := do
  -- u ← instantiateLevelMVars u
  visitLevel collectLevelAux u

def preprocess (e : Expr) : ClosureM Expr := do
  let e ← instantiateMVars e
  let ctx ← read
  -- If we are not zetaDelta-expanding let-decls, then we use `check` to find
  -- which let-decls are dependent. We say a let-decl is dependent if its lambda abstraction is type incorrect.
  if !ctx.zetaDelta then
    check e
  pure e

/--
  Remark: This method does not guarantee unique user names.
  The correctness of the procedure does not rely on unique user names.
  Recall that the pretty printer takes care of unintended collisions. -/
def mkNextUserName : ClosureM Name := do
  let s ← get
  let n := (`_x).appendIndexAfter s.nextExprIdx
  modify fun s => { s with nextExprIdx := s.nextExprIdx + 1 }
  pure n

def pushToProcess (elem : ToProcessElement) : ClosureM Unit :=
  modify fun s => { s with toProcess := s.toProcess.push elem }

partial def collectExprAux (e : Expr) : ClosureM Expr := do
  let collect (e : Expr) := visitExpr collectExprAux e
  match e with
  | Expr.proj _ _ s      => return e.updateProj! (← collect s)
  | Expr.forallE _ d b _ => return e.updateForallE! (← collect d) (← collect b)
  | Expr.lam _ d b _     => return e.updateLambdaE! (← collect d) (← collect b)
  | Expr.letE _ t v b _  => return e.updateLet! (← collect t) (← collect v) (← collect b)
  | Expr.app f a         => return e.updateApp! (← collect f) (← collect a)
  | Expr.mdata _ b       => return e.updateMData! (← collect b)
  | Expr.sort u          => return e.updateSort! (← collectLevel u)
  | Expr.const _ us      => return e.updateConst! (← us.mapM collectLevel)
  | Expr.mvar mvarId     =>
    let mvarDecl ← mvarId.getDecl
    let type ← preprocess mvarDecl.type
    let type ← collect type
    let newFVarId ← mkFreshFVarId
    let userName ← mkNextUserName
    /-
    Recall that delayed assignment metavariables must always be applied to at least
    `a.fvars.size` arguments (where `a : DelayedMetavarAssignment` is its record).
    This assumption is used in `lean::instantiate_mvars_fn::visit_app` for example, where there's a comment
    about how under-applied delayed assignments are an error.

    If we were to collect the delayed assignment metavariable itself and push it onto the `exprMVarArgs` list,
    then `exprArgs` returned by `Lean.Meta.Closure.mkValueTypeClosure` would contain underapplied delayed assignment metavariables.
    This leads to kernel 'declaration has metavariables' errors, as reported in https://github.com/leanprover/lean4/issues/6354

    The straightforward solution to this problem (implemented below) is to eta expand the delayed assignment metavariable
    to ensure it is fully applied. This isn't full eta expansion; we only need to eta expand the first `fvars.size` arguments.

    Note: there is the possibility of handling special cases to create more-efficient terms.
    For example, if the delayed assignment metavariable is applied to fvars, we could avoid eta expansion for those arguments
    since the fvars are being collected anyway. It's not clear that the additional implementation complexity is worth it,
    and it is something we can evaluate later. In any case, the current solution is necessary as the generic case.
    -/
    let e' ←
      if let some { fvars, .. } ← getDelayedMVarAssignment? mvarId then
        -- Eta expand `e` for the requisite number of arguments.
        forallBoundedTelescope mvarDecl.type fvars.size fun args _ => do
          mkLambdaFVars args <| mkAppN e args
      else
        pure e
    modify fun s => { s with
      newLocalDeclsForMVars := s.newLocalDeclsForMVars.push $ .cdecl default newFVarId userName type .default .default,
      exprMVarArgs          := s.exprMVarArgs.push e'
    }
    return mkFVar newFVarId
  | Expr.fvar fvarId =>
    match (← read).zetaDelta, (← fvarId.getValue?) with
    | true, some value => collect (← preprocess value)
    | _,    _          =>
      let newFVarId ← mkFreshFVarId
      pushToProcess ⟨fvarId, newFVarId⟩
      return mkFVar newFVarId
  | e => pure e

def collectExpr (e : Expr) : ClosureM Expr := do
  let e ← preprocess e
  visitExpr collectExprAux e

partial def pickNextToProcessAux (lctx : LocalContext) (i : Nat) (toProcess : Array ToProcessElement) (elem : ToProcessElement)
    : ToProcessElement × Array ToProcessElement :=
  if h : i < toProcess.size then
    let elem' := toProcess[i]
    if (lctx.get! elem.fvarId).index < (lctx.get! elem'.fvarId).index then
      pickNextToProcessAux lctx (i+1) (toProcess.set i elem) elem'
    else
      pickNextToProcessAux lctx (i+1) toProcess elem
  else
    (elem, toProcess)

def pickNextToProcess? : ClosureM (Option ToProcessElement) := do
  let lctx ← getLCtx
  let s ← get
  if s.toProcess.isEmpty then
    pure none
  else
    modifyGet fun s =>
      let elem      := s.toProcess.back!
      let toProcess := s.toProcess.pop
      let (elem, toProcess) := pickNextToProcessAux lctx 0 toProcess elem
      (some elem, { s with toProcess := toProcess })

def pushFVarArg (e : Expr) : ClosureM Unit :=
  modify fun s => { s with exprFVarArgs := s.exprFVarArgs.push e }

def pushLocalDecl (newFVarId : FVarId) (userName : Name) (type : Expr) (bi := BinderInfo.default) : ClosureM Unit := do
  let type ← collectExpr type
  modify fun s => { s with newLocalDecls := s.newLocalDecls.push <| .cdecl default newFVarId userName type bi .default }

partial def process : ClosureM Unit := do
  match (← pickNextToProcess?) with
  | none => pure ()
  | some ⟨fvarId, newFVarId⟩ =>
    match (← fvarId.getDecl) with
    | .cdecl _ _ userName type bi _ =>
      pushLocalDecl newFVarId userName type bi
      pushFVarArg (mkFVar fvarId)
      process
    | .ldecl _ _ userName type val _ _ =>
      let zetaDeltaFVarIds ← getZetaDeltaFVarIds
      if !zetaDeltaFVarIds.contains fvarId then
        /- Non-dependent let-decl

            Recall that if `fvarId` is in `zetaDeltaFVarIds`, then we zetaDelta-expanded it
            during type checking (see `check` at `collectExpr`).

            Our type checker may zetaDelta-expand declarations that are not needed, but this
            check is conservative, and seems to work well in practice. -/
        pushLocalDecl newFVarId userName type
        pushFVarArg (mkFVar fvarId)
        process
      else
        /- Dependent let-decl -/
        let type ← collectExpr type
        let val  ← collectExpr val
        modify fun s => { s with newLetDecls := s.newLetDecls.push <| .ldecl default newFVarId userName type val false .default }
        /- We don't want to interleave let and lambda declarations in our closure. So, we expand any occurrences of newFVarId
           at `newLocalDecls` -/
        modify fun s => { s with newLocalDecls := s.newLocalDecls.map (·.replaceFVarId newFVarId val) }
        process

@[inline] def mkBinding (isLambda : Bool) (decls : Array LocalDecl) (b : Expr) : Expr :=
  let xs := decls.map LocalDecl.toExpr
  let b  := b.abstract xs
  decls.size.foldRev (init := b) fun i _ b =>
    let decl := decls[i]
    match decl with
    | .cdecl _ _ n ty bi _ =>
      let ty := ty.abstractRange i xs
      if isLambda then
        Lean.mkLambda n bi ty b
      else
        Lean.mkForall n bi ty b
    | .ldecl _ _ n ty val nonDep _ =>
      if b.hasLooseBVar 0 then
        let ty  := ty.abstractRange i xs
        let val := val.abstractRange i xs
        mkLet n ty val b nonDep
      else
        b.lowerLooseBVars 1 1

def mkLambda (decls : Array LocalDecl) (b : Expr) : Expr :=
  mkBinding true decls b

def mkForall (decls : Array LocalDecl) (b : Expr) : Expr :=
  mkBinding false decls b

structure MkValueTypeClosureResult where
  levelParams : Array Name
  type        : Expr
  value       : Expr
  levelArgs   : Array Level
  exprArgs    : Array Expr

def mkValueTypeClosureAux (type : Expr) (value : Expr) : ClosureM (Expr × Expr) := do
  resetZetaDeltaFVarIds
  withTrackingZetaDelta do
    let type  ← collectExpr type
    let value ← collectExpr value
    process
    pure (type, value)

def mkValueTypeClosure (type : Expr) (value : Expr) (zetaDelta : Bool) : MetaM MkValueTypeClosureResult := do
  let ((type, value), s) ← ((mkValueTypeClosureAux type value).run { zetaDelta }).run {}
  let newLocalDecls := s.newLocalDecls.reverse ++ s.newLocalDeclsForMVars
  let newLetDecls   := s.newLetDecls.reverse
  let type  := mkForall newLocalDecls (mkForall newLetDecls type)
  let value := mkLambda newLocalDecls (mkLambda newLetDecls value)
  pure {
    type        := type,
    value       := value,
    levelParams := s.levelParams,
    levelArgs   := s.levelArgs,
    exprArgs    := s.exprFVarArgs.reverse ++ s.exprMVarArgs
  }

end Closure

/--
  Create an auxiliary definition with the given name, type and value.
  The parameters `type` and `value` may contain free and meta variables.
  A "closure" is computed, and a term of the form `name.{u_1 ... u_n} t_1 ... t_m` is
  returned where `u_i`s are universe parameters and metavariables `type` and `value` depend on,
  and `t_j`s are free and meta variables `type` and `value` depend on. -/
def mkAuxDefinition (name : Name) (type : Expr) (value : Expr) (zetaDelta : Bool := false) (compile : Bool := true) : MetaM Expr := do
  let result ← Closure.mkValueTypeClosure type value zetaDelta
  let env ← getEnv
  let hints := ReducibilityHints.regular (getMaxHeight env result.value + 1)
  let decl := Declaration.defnDecl (← mkDefinitionValInferrringUnsafe name result.levelParams.toList
    result.type result.value  hints)
  addDecl decl
  if compile then
    compileDecl decl
  return mkAppN (mkConst name result.levelArgs.toList) result.exprArgs

/-- Similar to `mkAuxDefinition`, but infers the type of `value`. -/
def mkAuxDefinitionFor (name : Name) (value : Expr) (zetaDelta : Bool := false) : MetaM Expr := do
  let type ← inferType value
  let type := type.headBeta
  mkAuxDefinition name type value (zetaDelta := zetaDelta)

/--
  Create an auxiliary theorem with the given name, type and value. It is similar to `mkAuxDefinition`.
-/
def mkAuxTheorem (type : Expr) (value : Expr) (zetaDelta : Bool := false) (prefix? : Option Name) (cache := true) : MetaM Expr := do
  let result ← Closure.mkValueTypeClosure type value zetaDelta
  let name ← mkAuxLemma (prefix? := prefix?) (cache := cache) result.levelParams.toList result.type result.value
  return mkAppN (mkConst name result.levelArgs.toList) result.exprArgs

end Lean.Meta