/-
Copyright (c) 2021 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Leonardo de Moura
-/
prelude
import Lean.Util.CollectLevelParams
import Lean.Util.CollectAxioms
import Lean.Meta.Reduce
import Lean.Elab.DeclarationRange
import Lean.Elab.Eval
import Lean.Elab.Command
import Lean.Elab.Open
import Lean.Elab.SetOption
import Init.System.Platform

namespace Lean.Elab.Command

@[builtin_command_elab moduleDoc] def elabModuleDoc : CommandElab := fun stx => do
  match stx[1] with
  | Syntax.atom _ val =>
    let doc := val.extract 0 (val.endPos - ⟨2⟩)
    let some range ← Elab.getDeclarationRange? stx
      | return  -- must be from partial syntax, ignore
    modifyEnv fun env => addMainModuleDoc env ⟨doc, range⟩
  | _ => throwErrorAt stx "unexpected module doc string{indentD stx[1]}"

private def addScope (isNewNamespace : Bool) (isNoncomputable : Bool) (header : String) (newNamespace : Name) : CommandElabM Unit := do
  modify fun s => { s with
    env    := s.env.registerNamespace newNamespace,
    scopes := { s.scopes.head! with header := header, currNamespace := newNamespace, isNoncomputable := s.scopes.head!.isNoncomputable || isNoncomputable } :: s.scopes
  }
  pushScope
  if isNewNamespace then
    activateScoped newNamespace

private def addScopes (isNewNamespace : Bool) (isNoncomputable : Bool) : Name → CommandElabM Unit
  | .anonymous => pure ()
  | .str p header => do
    addScopes isNewNamespace isNoncomputable p
    let currNamespace ← getCurrNamespace
    addScope isNewNamespace isNoncomputable header (if isNewNamespace then Name.mkStr currNamespace header else currNamespace)
  | _ => throwError "invalid scope"

private def addNamespace (header : Name) : CommandElabM Unit :=
  addScopes (isNewNamespace := true) (isNoncomputable := false) header

def withNamespace {α} (ns : Name) (elabFn : CommandElabM α) : CommandElabM α := do
  addNamespace ns
  let a ← elabFn
  modify fun s => { s with scopes := s.scopes.drop ns.getNumParts }
  pure a

private def popScopes (numScopes : Nat) : CommandElabM Unit :=
  for _ in [0:numScopes] do
    popScope

private def checkAnonymousScope : List Scope → Option Name
  | { header := "", .. } :: _ => none
  | { header := h, .. }  :: _ => some <| .mkSimple h
  | _                         => some .anonymous -- should not happen

private def checkEndHeader : Name → List Scope → Option Name
  | .anonymous, _ => none
  | .str p s, { header := h, .. } :: scopes =>
    if h == s then
      (.str · s) <$> checkEndHeader p scopes
    else
      some <| .mkSimple h
  | _, _ => some .anonymous -- should not happen

@[builtin_command_elab «namespace»] def elabNamespace : CommandElab := fun stx =>
  match stx with
  | `(namespace $n) => addNamespace n.getId
  | _               => throwUnsupportedSyntax

@[builtin_command_elab «section»] def elabSection : CommandElab := fun stx => do
  match stx with
  | `(section $header:ident) => addScopes (isNewNamespace := false) (isNoncomputable := false) header.getId
  | `(section)               => addScope (isNewNamespace := false) (isNoncomputable := false) "" (← getCurrNamespace)
  | _                        => throwUnsupportedSyntax

@[builtin_command_elab noncomputableSection] def elabNonComputableSection : CommandElab := fun stx => do
  match stx with
  | `(noncomputable section $header:ident) => addScopes (isNewNamespace := false) (isNoncomputable := true) header.getId
  | `(noncomputable section)               => addScope (isNewNamespace := false) (isNoncomputable := true) "" (← getCurrNamespace)
  | _                        => throwUnsupportedSyntax

@[builtin_command_elab «end»] def elabEnd : CommandElab := fun stx => do
  let header? := (stx.getArg 1).getOptionalIdent?;
  let endSize := match header? with
    | none   => 1
    | some n => n.getNumParts
  let scopes ← getScopes
  if endSize < scopes.length then
    modify fun s => { s with scopes := s.scopes.drop endSize }
    popScopes endSize
  else -- we keep "root" scope
    let n := (← get).scopes.length - 1
    modify fun s => { s with scopes := s.scopes.drop n }
    popScopes n
    throwError "invalid 'end', insufficient scopes"
  match header? with
  | none        =>
    if let some name := checkAnonymousScope scopes then
      throwError "invalid 'end', name is missing (expected {name})"
  | some header =>
    if let some name := checkEndHeader header scopes then
      addCompletionInfo <| CompletionInfo.endSection stx (scopes.map fun scope => scope.header)
      throwError "invalid 'end', name mismatch (expected {if name == `«» then `nothing else name})"

private partial def elabChoiceAux (cmds : Array Syntax) (i : Nat) : CommandElabM Unit :=
  if h : i < cmds.size then
    catchInternalId unsupportedSyntaxExceptionId
      (elabCommand cmds[i])
      (fun _ => elabChoiceAux cmds (i+1))
  else
    throwUnsupportedSyntax

@[builtin_command_elab choice] def elabChoice : CommandElab := fun stx =>
  elabChoiceAux stx.getArgs 0

@[builtin_command_elab «universe»] def elabUniverse : CommandElab := fun n => do
  n[1].forArgsM addUnivLevel

@[builtin_command_elab «init_quot»] def elabInitQuot : CommandElab := fun _ => do
  liftCoreM <| addDecl Declaration.quotDecl

@[builtin_command_elab «export»] def elabExport : CommandElab := fun stx => do
  let `(export $ns ($ids*)) := stx | throwUnsupportedSyntax
  let nss ← resolveNamespace ns
  let currNamespace ← getCurrNamespace
  if nss == [currNamespace] then throwError "invalid 'export', self export"
  let mut aliases := #[]
  for idStx in ids do
    let id := idStx.getId
    let declName ← resolveNameUsingNamespaces nss idStx
    if (← getInfoState).enabled then
      addConstInfo idStx declName
    aliases := aliases.push (currNamespace ++ id, declName)
  modify fun s => { s with env := aliases.foldl (init := s.env) fun env p => addAlias env p.1 p.2 }

@[builtin_command_elab «open»] def elabOpen : CommandElab
  | `(open $decl:openDecl) => do
    let openDecls ← elabOpenDecl decl
    modifyScope fun scope => { scope with openDecls := openDecls }
  | _ => throwUnsupportedSyntax

open Lean.Parser.Term

private def typelessBinder? : Syntax → Option (Array (TSyntax [`ident, `Lean.Parser.Term.hole]) × BinderInfo)
  | `(bracketedBinderF|($ids*))     => some (ids, .default)
  | `(bracketedBinderF|{$ids*})     => some (ids, .implicit)
  | `(bracketedBinderF|⦃$ids*⦄)     => some (ids, .strictImplicit)
  | `(bracketedBinderF|[$id:ident]) => some (#[id], .instImplicit)
  | _                               => none

/--  If `id` is an identifier, return true if `ids` contains `id`. -/
private def containsId (ids : Array (TSyntax [`ident, ``Parser.Term.hole])) (id : TSyntax [`ident, ``Parser.Term.hole]) : Bool :=
  id.raw.isIdent && ids.any fun id' => id'.raw.getId == id.raw.getId

/--
  Auxiliary method for processing binder annotation update commands:
  `variable (α)`, `variable {α}`, `variable ⦃α⦄`, and `variable [α]`.
  The argument `binder` is the binder of the `variable` command.
  The method returns an array containing the "residue", that is, variables that do not correspond to updates.
  Recall that a `bracketedBinder` can be of the form `(x y)`.
  ```
  variable {α β : Type}
  variable (α γ)
  ```
  The second `variable` command updates the binder annotation for `α`, and returns "residue" `γ`.
-/
private def replaceBinderAnnotation (binder : TSyntax ``Parser.Term.bracketedBinder) : CommandElabM (Array (TSyntax ``Parser.Term.bracketedBinder)) := do
  let some (binderIds, binderInfo) := typelessBinder? binder | return #[binder]
  let varDecls := (← getScope).varDecls
  let mut varDeclsNew := #[]
  let mut binderIds := binderIds
  let mut binderIdsIniSize := binderIds.size
  let mut modifiedVarDecls := false
  -- Go through declarations in reverse to respect shadowing
  for varDecl in varDecls.reverse do
    let (ids, ty?, binderInfo') ← match varDecl with
      | `(bracketedBinderF|($ids* $[: $ty?]? $(annot?)?)) =>
        if annot?.isSome then
          for binderId in binderIds do
            if containsId ids binderId then
              throwErrorAt binderId "cannot update binder annotation of variables with default values/tactics"
        pure (ids, ty?, .default)
      | `(bracketedBinderF|{$ids* $[: $ty?]?}) =>
        pure (ids, ty?, .implicit)
      | `(bracketedBinderF|⦃$ids* $[: $ty?]?⦄) =>
        pure (ids, ty?, .strictImplicit)
      | `(bracketedBinderF|[$id : $ty]) =>
        pure (#[⟨id⟩], some ty, .instImplicit)
      | _ =>
        varDeclsNew := varDeclsNew.push varDecl; continue
    if binderInfo == binderInfo' then
      -- no update, ensure we don't have redundant annotations.
      for binderId in binderIds do
        if containsId ids binderId then
          throwErrorAt binderId "redundant binder annotation update"
      varDeclsNew := varDeclsNew.push varDecl
    else if binderIds.all fun binderId => !containsId ids binderId then
      -- `binderIds` and `ids` are disjoint
      varDeclsNew := varDeclsNew.push varDecl
    else
      let mkBinder (id : TSyntax [`ident, ``Parser.Term.hole]) (binderInfo : BinderInfo) : CommandElabM (TSyntax ``Parser.Term.bracketedBinder) :=
        match binderInfo with
        | .default => `(bracketedBinderF| ($id $[: $ty?]?))
        | .implicit => `(bracketedBinderF| {$id $[: $ty?]?})
        | .strictImplicit => `(bracketedBinderF| {{$id $[: $ty?]?}})
        | .instImplicit => do
          let some ty := ty?
            | throwErrorAt binder "cannot update binder annotation of variable '{id}' to instance implicit:\n\
                variable was originally declared without an explicit type"
          `(bracketedBinderF| [$(⟨id⟩) : $ty])
      for id in ids.reverse do
        if let some idx := binderIds.findFinIdx? fun binderId => binderId.raw.isIdent && binderId.raw.getId == id.raw.getId then
          binderIds := binderIds.eraseIdx idx
          modifiedVarDecls := true
          let newBinder ← mkBinder id binderInfo
          if binderInfo.isInstImplicit then
            -- We elaborate the new binder to make sure it's valid as instance implicit
            try
              runTermElabM fun _ => Term.withSynthesize <| Term.withAutoBoundImplicit <|
                Term.elabBinder newBinder fun _ => pure ()
            catch e =>
              throwErrorAt binder m!"cannot update binder annotation of variable '{id}' to instance implicit:\n\
                {e.toMessageData}"
          varDeclsNew := varDeclsNew.push (← mkBinder id binderInfo)
        else
          varDeclsNew := varDeclsNew.push (← mkBinder id binderInfo')
  if modifiedVarDecls then
    modifyScope fun scope => { scope with varDecls := varDeclsNew.reverse }
  if binderIds.size != binderIdsIniSize then
    binderIds.mapM fun binderId =>
      match binderInfo with
        | .default => `(bracketedBinderF| ($binderId))
        | .implicit => `(bracketedBinderF| {$binderId})
        | .strictImplicit => `(bracketedBinderF| {{$binderId}})
        | .instImplicit => throwUnsupportedSyntax
  else
    return #[binder]

@[builtin_command_elab «variable»] def elabVariable : CommandElab
  | `(variable%$tk $binders*) => do
    let binders ← binders.flatMapM replaceBinderAnnotation
    -- Try to elaborate `binders` for sanity checking
    runTermElabM fun _ => Term.withSynthesize <| Term.withAutoBoundImplicit <|
      Term.elabBinders binders fun xs => do
        -- Determine the set of auto-implicits for this variable command and add an inlay hint
        -- for them. We will only actually add the auto-implicits to a type when the variables
        -- declared here are used in some other declaration, but this is nonetheless the right
        -- place to display the inlay hint.
        let _ ← Term.addAutoBoundImplicits xs (tk.getTailPos? (canonicalOnly := true))
    -- Remark: if we want to produce error messages when variables shadow existing ones, here is the place to do it.
    for binder in binders do
      let varUIds ← (← getBracketedBinderIds binder) |>.mapM (withFreshMacroScope ∘ MonadQuotation.addMacroScope)
      modifyScope fun scope => { scope with varDecls := scope.varDecls.push binder, varUIds := scope.varUIds ++ varUIds }
  | _ => throwUnsupportedSyntax

open Meta

def elabCheckCore (ignoreStuckTC : Bool) : CommandElab
  | `(#check%$tk $term) => withoutModifyingEnv <| runTermElabM fun _ => Term.withDeclName `_check do
    -- show signature for `#check id`/`#check @id`
    if let `($id:ident) := term then
      try
        for c in (← realizeGlobalConstWithInfos term) do
          addCompletionInfo <| .id term id.getId (danglingDot := false) {} none
          logInfoAt tk <| .signature c
          return
      catch _ => pure ()  -- identifier might not be a constant but constant + projection
    let e ← Term.elabTerm term none
    Term.synthesizeSyntheticMVarsNoPostponing (ignoreStuckTC := ignoreStuckTC)
    -- Users might be testing out buggy elaborators. Let's typecheck before proceeding:
    withRef tk <| Meta.check e
    let e ← Term.levelMVarToParam (← instantiateMVars e)
    if e.isSyntheticSorry then
      return
    let type ← inferType e
    logInfoAt tk m!"{e} : {type}"
  | _ => throwUnsupportedSyntax

@[builtin_command_elab Lean.Parser.Command.check] def elabCheck : CommandElab := elabCheckCore (ignoreStuckTC := true)

@[builtin_command_elab Lean.reduceCmd] def elabReduce : CommandElab
  | `(#reduce%$tk $term) => go tk term
  | `(#reduce%$tk (proofs := true) $term) => go tk term (skipProofs := false)
  | `(#reduce%$tk (types := true) $term) => go tk term (skipTypes := false)
  | `(#reduce%$tk (proofs := true) (types := true) $term) => go tk term (skipProofs := false) (skipTypes := false)
  | _ => throwUnsupportedSyntax
where
  go (tk : Syntax) (term : Syntax) (skipProofs := true) (skipTypes := true) : CommandElabM Unit :=
    withoutModifyingEnv <| runTermElabM fun _ => Term.withDeclName `_reduce do
      let e ← Term.elabTerm term none
      Term.synthesizeSyntheticMVarsNoPostponing
      -- Users might be testing out buggy elaborators. Let's typecheck before proceeding:
      withRef tk <| Meta.check e
      let e ← Term.levelMVarToParam (← instantiateMVars e)
      -- TODO: add options or notation for setting the following parameters
      withTheReader Core.Context (fun ctx => { ctx with options := ctx.options.setBool `smartUnfolding false }) do
        let e ← withTransparency (mode := TransparencyMode.all) <| reduce e (skipProofs := skipProofs) (skipTypes := skipTypes)
        logInfoAt tk e

def hasNoErrorMessages : CommandElabM Bool := do
  return !(← get).messages.hasErrors

def failIfSucceeds (x : CommandElabM Unit) : CommandElabM Unit := do
  let resetMessages : CommandElabM MessageLog := do
    let s ← get
    let messages := s.messages;
    modify fun s => { s with messages := {} };
    pure messages
  let restoreMessages (prevMessages : MessageLog) : CommandElabM Unit := do
    modify fun s => { s with messages := prevMessages ++ s.messages.errorsToInfos }
  let prevMessages ← resetMessages
  let succeeded ← try
    x
    hasNoErrorMessages
  catch
    | ex@(Exception.error _ _) => do logException ex; pure false
    | Exception.internal id _  => do logError (← id.getName); pure false
  finally
    restoreMessages prevMessages
  if succeeded then
    throwError "unexpected success"

@[builtin_command_elab «check_failure»] def elabCheckFailure : CommandElab
  | `(#check_failure $term) => do
    failIfSucceeds <| elabCheckCore (ignoreStuckTC := false) (← `(#check $term))
  | _ => throwUnsupportedSyntax

@[builtin_command_elab «synth»] def elabSynth : CommandElab := fun stx => do
  let term := stx[1]
  withoutModifyingEnv <| runTermElabM fun _ => Term.withDeclName `_synth_cmd do
    let inst ← Term.elabTerm term none
    Term.synthesizeSyntheticMVarsNoPostponing
    let inst ← instantiateMVars inst
    let val  ← synthInstance inst
    logInfo val
    pure ()

@[builtin_command_elab «set_option»] def elabSetOption : CommandElab := fun stx => do
  let options ← Elab.elabSetOption stx[1] stx[3]
  modify fun s => { s with maxRecDepth := maxRecDepth.get options }
  modifyScope fun scope => { scope with opts := options }

@[builtin_macro Lean.Parser.Command.«in»] def expandInCmd : Macro
  | `($cmd₁ in%$tk $cmd₂) =>
    -- Limit ref variability for incrementality; see Note [Incremental Macros]
    withRef tk `(section $cmd₁:command $cmd₂ end)
  | _                 => Macro.throwUnsupported

@[builtin_command_elab Parser.Command.addDocString] def elabAddDeclDoc : CommandElab := fun stx => do
  match stx with
  | `($doc:docComment add_decl_doc $id) =>
    let declName ← liftCoreM <| realizeGlobalConstNoOverloadWithInfo id
    unless ((← getEnv).getModuleIdxFor? declName).isNone do
      throwError "invalid 'add_decl_doc', declaration is in an imported module"
    if let .none ← findDeclarationRangesCore? declName then
      -- this is only relevant for declarations added without a declaration range
      -- in particular `Quot.mk` et al which are added by `init_quot`
      addDeclarationRangesFromSyntax declName stx id
    addDocString declName (← getDocStringText doc)
  | _ => throwUnsupportedSyntax

@[builtin_command_elab Lean.Parser.Command.include] def elabInclude : CommandElab
  | `(Lean.Parser.Command.include| include $ids*) => do
    let sc ← getScope
    let vars ← sc.varDecls.flatMapM getBracketedBinderIds
    let mut uids := #[]
    for id in ids do
      if let some idx := vars.findIdx? (· == id.getId) then
        uids := uids.push sc.varUIds[idx]!
      else
        throwError "invalid 'include', variable '{id}' has not been declared in the current scope"
    modifyScope fun sc => { sc with
      includedVars := sc.includedVars ++ uids.toList
      omittedVars := sc.omittedVars.filter (!uids.contains ·) }
  | _ => throwUnsupportedSyntax

@[builtin_command_elab Lean.Parser.Command.omit] def elabOmit : CommandElab
  | `(Lean.Parser.Command.omit| omit $omits*) => do
    -- TODO: this really shouldn't have to re-elaborate section vars... they should come
    -- pre-elaborated
    let omittedVars ← runTermElabM fun vars => do
      Term.synthesizeSyntheticMVarsNoPostponing
      -- We don't want to store messages produced when elaborating `(getVarDecls s)` because they have already been saved when we elaborated the `variable`(s) command.
      -- So, we use `Core.resetMessageLog`.
      Core.resetMessageLog
      -- resolve each omit to variable user name or type pattern
      let elaboratedOmits : Array (Sum Name Expr) ← omits.mapM fun
        | `(ident| $id:ident) => pure <| Sum.inl id.getId
        | `(Lean.Parser.Term.instBinder| [$id : $_]) => pure <| Sum.inl id.getId
        | `(Lean.Parser.Term.instBinder| [$ty]) =>
          Sum.inr <$> Term.withoutErrToSorry (Term.elabTermAndSynthesize ty none)
        | _ => throwUnsupportedSyntax
      -- check that each omit is actually used in the end
      let mut omitsUsed := omits.map fun _ => false
      let mut omittedVars := #[]
      let mut revSectionFVars : Std.HashMap FVarId Name := {}
      for (uid, var) in (← read).sectionFVars do
        revSectionFVars := revSectionFVars.insert var.fvarId! uid
      for var in vars do
        let ldecl ← var.fvarId!.getDecl
        if let some idx := (← elaboratedOmits.findIdxM? fun
            | .inl id => return ldecl.userName == id
            | .inr ty => do
              let mctx ← getMCtx
              isDefEq ty ldecl.type <* setMCtx mctx) then
          if let some uid := revSectionFVars[var.fvarId!]? then
            omittedVars := omittedVars.push uid
            omitsUsed := omitsUsed.set! idx true
          else
            throwError "invalid 'omit', '{ldecl.userName}' has not been declared in the current scope"
      for o in omits, used in omitsUsed do
        unless used do
          throwError "'{o}' did not match any variables in the current scope"
      return omittedVars
    modifyScope fun sc => { sc with
      omittedVars := sc.omittedVars ++ omittedVars.toList
      includedVars := sc.includedVars.filter (!omittedVars.contains ·) }
  | _ => throwUnsupportedSyntax

@[builtin_command_elab version] def elabVersion : CommandElab := fun _ => do
  let mut target := System.Platform.target
  if target.isEmpty then target := "unknown"
  -- Only one should be set, but good to know if multiple are set in error.
  let platforms :=
    (if System.Platform.isWindows then [" Windows"] else [])
    ++ (if System.Platform.isOSX then [" macOS"] else [])
    ++ (if System.Platform.isEmscripten then [" Emscripten"] else [])
  logInfo m!"Lean {Lean.versionString}\nTarget: {target}{String.join platforms}"

@[builtin_command_elab Parser.Command.exit] def elabExit : CommandElab := fun _ =>
  logWarning "using 'exit' to interrupt Lean"

@[builtin_command_elab Parser.Command.import] def elabImport : CommandElab := fun _ =>
  throwError "invalid 'import' command, it must be used in the beginning of the file"

@[builtin_command_elab Parser.Command.eoi] def elabEoi : CommandElab := fun _ =>
  return

@[builtin_command_elab Parser.Command.where] def elabWhere : CommandElab := fun _ => do
  let scope ← getScope
  let mut msg : Array MessageData := #[]
  -- Noncomputable
  if scope.isNoncomputable then
    msg := msg.push <| ← `(command| noncomputable section)
  -- Namespace
  if !scope.currNamespace.isAnonymous then
    msg := msg.push <| ← `(command| namespace $(mkIdent scope.currNamespace))
  -- Open namespaces
  if let some openMsg ← describeOpenDecls scope.openDecls.reverse then
    msg := msg.push openMsg
  -- Universe levels
  if !scope.levelNames.isEmpty then
    let levels := scope.levelNames.reverse.map mkIdent
    msg := msg.push <| ← `(command| universe $levels.toArray*)
  -- Variables
  if !scope.varDecls.isEmpty then
    let varDecls : Array (TSyntax `Lean.Parser.Term.bracketedBinder) := scope.varDecls.map (⟨·.raw.unsetTrailing⟩)
    msg := msg.push <| ← `(command| variable $varDecls*)
  -- Included variables
  if !scope.includedVars.isEmpty then
    msg := msg.push <| ← `(command| include $(scope.includedVars.toArray.map (mkIdent ·.eraseMacroScopes))*)
  -- Options
  if let some optionsMsg ← describeOptions scope.opts then
    msg := msg.push optionsMsg
  if msg.isEmpty then
    logInfo m!"-- In root namespace with initial scope"
  else
    logInfo <| MessageData.joinSep msg.toList "\n\n"
where
  /--
  'Delaborate' open declarations.
  Current limitations:
  - does not check whether or not successive namespaces need `_root_`
  - does not combine commands with `renaming` clauses into a single command
  -/
  describeOpenDecls (ds : List OpenDecl) : CommandElabM (Option MessageData) := do
    let mut lines : Array MessageData := #[]
    let mut simple : Array Name := #[]
    let flush (lines : Array MessageData) (simple : Array Name) : CommandElabM (Array MessageData × Array Name) := do
      if simple.isEmpty then
        return (lines, simple)
      else
        return (lines.push <| ← `(command| open $(simple.map mkIdent)*), #[])
    for d in ds do
      match d with
      | .explicit id decl =>
        (lines, simple) ← flush lines simple
        let ns := decl.getPrefix
        let «from» := Name.mkSimple decl.getString!
        lines := lines.push <| ← `(command| open $(mkIdent ns) renaming $(mkIdent «from») → $(mkIdent id))
      | .simple ns ex =>
        if ex == [] then
          simple := simple.push ns
        else
          (lines, simple) ← flush lines simple
          lines := lines.push <| ← `(command| open $(mkIdent ns) hiding $[$(ex.toArray.map mkIdent)]*)
    (lines, _) ← flush lines simple
    return if lines.isEmpty then none else MessageData.joinSep lines.toList "\n"

  describeOptions (opts : Options) : CommandElabM (Option MessageData) := do
    let mut lines : Array MessageData := #[]
    let decls ← getOptionDecls
    for (name, val) in opts do
      -- `#guard_msgs` sets this option internally, we don't want it to end up in its output
      if name == `Elab.async then
        continue
      let (isSet, isUnknown) :=
        match decls.find? name with
        | some decl => (decl.defValue != val, false)
        | none      => (true, true)
      if isSet then
        let cmd : TSyntax `command ←
          match val with
          | .ofBool true  => `(set_option $(mkIdent name) true)
          | .ofBool false => `(set_option $(mkIdent name) false)
          | .ofString str => `(set_option $(mkIdent name) $(Syntax.mkStrLit str))
          | .ofNat n      => `(set_option $(mkIdent name) $(Syntax.mkNatLit n))
          | _             => `(set_option $(mkIdent name) 0 /- unrepresentable value -/)
        if isUnknown then
          lines := lines.push m!"-- {cmd} -- unknown option"
        else
          lines := lines.push cmd
    return if lines.isEmpty then none else MessageData.joinSep lines.toList "\n"

end Lean.Elab.Command
