mercury/compiler/lambda.m

%-----------------------------------------------------------------------------%
% vim: ft=mercury ts=4 sw=4 et
%-----------------------------------------------------------------------------%
% Copyright (C) 1995-2012 The University of Melbourne.
% This file may only be copied under the terms of the GNU General
% Public License - see the file COPYING in the Mercury distribution.
%-----------------------------------------------------------------------------%
%
% File: lambda.m.
% Main author: fjh.
%
% This module does lambda expansion, which means that it replaces each
% unification with a lambda expression with the construction of a closure
% whose code address refers to a new predicate that this module creates
% from that lambda expression.
%
% For example, we translate
%
%   :- pred p(int::in) is det.
%   p(X) :-
%       V__1 = (pred(Y::out) is nondet :- q(Y, X)),
%       solutions(V__1, List),
%       ...
%   :- pred q(int::out, int::in) is nondet.
%
% into
%
%   :- pred '__LambdaGoal__1'(int::in, int::out) is nondet.
%   '__LambdaGoal__1'(X, Y) :- q(Y, X).
%
%   p(X) :-
%       V__1 = '__LambdaGoal__1'(X)
%       solutions(V__1, List),
%       ...
%
%
% Note that the mode checker requires that lambda expressions
% not bind any of their non-local variables, such as `X' in the above example.
%
% Similarly, a lambda expression may not bind any of the type_infos for
% those variables; that is, none of the non-local variables should be
% existentially typed (from the perspective of the lambda goal).
% Now that we run the polymorphism.m pass before mode checking,
% and that this is also checked by mode analysis.
%
% It might be OK to allow the parameters of the lambda goal to be
% existentially typed, but currently that is not supported.
% One difficulty is that it's hard to determine here which type variables
% should be existentially quantified. The information is readily
% available during type inference, and really type inference should save
% that information in a field in the lambda_goal struct, but currently it
% doesn't; it saves the head_type_params field in the pred_info, which
% tells us which type variables were produced by the body, but for
% any given lambda goal, we don't know whether the type variable was
% produced by something outside the lambda goal or by something inside
% the lambda goal (only in the latter case should it be existentially
% quantified).
%
% The other difficulty is that taking the address of a predicate with an
% existential type would require second-order polymorphism: for a predicate
% declared as `:- some [T] pred p(int, T)', the expression `p' must have
% type `some [T] pred(int, T)', which is quite a different thing to saying
% that there is some type `T' for which `p' has type `pred(int, T)' --
% we don't know what `T' is until the predicate is called, and it might
% be different for each call.
%
% Currently we don't support second-order polymorphism, so we can't support
% existentially typed lambda expressions either.
%
%-----------------------------------------------------------------------------%

:- module transform_hlds.lambda.
:- interface.

:- import_module hlds.hlds_goal.
:- import_module hlds.hlds_module.
:- import_module hlds.hlds_pred.
:- import_module hlds.hlds_rtti.
:- import_module hlds.vartypes.
:- import_module mdbcomp.prim_data.
:- import_module parse_tree.prog_data.
:- import_module parse_tree.set_of_var.

:- import_module bool.
:- import_module list.

%-----------------------------------------------------------------------------%

:- pred expand_lambdas_in_module(module_info::in, module_info::out) is det.

:- pred expand_lambdas_in_pred(pred_id::in, module_info::in, module_info::out)
    is det.

%-----------------------------------------------------------------------------%

% The following are exported for float_reg.m.

:- type lambda_info.

:- type reg_wrapper_proc
    --->    reg_wrapper_proc(set_of_progvar)
    ;       not_reg_wrapper_proc.

:- pred init_lambda_info(prog_varset::in, vartypes::in, tvarset::in,
    inst_varset::in, rtti_varmaps::in, has_parallel_conj::in, pred_info::in,
    module_info::in, lambda_info::out) is det.

:- pred lambda_info_get_varset(lambda_info::in, prog_varset::out) is det.
:- pred lambda_info_get_vartypes(lambda_info::in, vartypes::out) is det.
:- pred lambda_info_get_tvarset(lambda_info::in, tvarset::out) is det.
:- pred lambda_info_get_rtti_varmaps(lambda_info::in, rtti_varmaps::out)
    is det.
:- pred lambda_info_get_inst_varset(lambda_info::in, inst_varset::out) is det.
:- pred lambda_info_get_pred_info(lambda_info::in, pred_info::out) is det.
:- pred lambda_info_get_module_info(lambda_info::in, module_info::out) is det.
:- pred lambda_info_get_recompute_nonlocals(lambda_info::in, bool::out) is det.

:- pred lambda_info_set_varset(prog_varset::in,
    lambda_info::in, lambda_info::out) is det.
:- pred lambda_info_set_vartypes(vartypes::in,
    lambda_info::in, lambda_info::out) is det.
:- pred lambda_info_set_module_info(module_info::in,
    lambda_info::in, lambda_info::out) is det.
:- pred lambda_info_set_recompute_nonlocals(bool::in,
    lambda_info::in, lambda_info::out) is det.

:- pred expand_lambda(purity::in, ho_groundness::in,
    pred_or_func::in, lambda_eval_method::in, reg_wrapper_proc::in,
    list(prog_var)::in, list(mer_mode)::in, determinism::in,
    list(prog_var)::in, hlds_goal::in, unification::in,
    unify_rhs::out, unification::out,
    lambda_info::in, lambda_info::out) is det.

%-----------------------------------------------------------------------------%
%-----------------------------------------------------------------------------%

:- implementation.

:- import_module check_hlds.mode_util.
:- import_module check_hlds.type_util.
:- import_module hlds.code_model.
:- import_module hlds.goal_util.
:- import_module hlds.pred_table.
:- import_module hlds.quantification.
:- import_module hlds.status.
:- import_module libs.globals.
:- import_module libs.options.
:- import_module parse_tree.prog_mode.
:- import_module parse_tree.prog_type.
:- import_module parse_tree.prog_util.

:- import_module assoc_list.
:- import_module array.
:- import_module int.
:- import_module map.
:- import_module maybe.
:- import_module pair.
:- import_module require.
:- import_module set.
:- import_module term.
:- import_module varset.

%-----------------------------------------------------------------------------%

:- type lambda_info
    --->    lambda_info(
                li_varset               :: prog_varset,
                li_vartypes             :: vartypes,
                li_tvarset              :: tvarset,
                li_inst_varset          :: inst_varset,
                li_rtti_varmaps         :: rtti_varmaps,
                li_has_parallel_conj    :: has_parallel_conj,
                li_pred_info            :: pred_info,
                li_module_info          :: module_info,

                li_recompute_nonlocals  :: bool,
                % True iff we need to recompute the nonlocals.

                li_have_expanded_lambda :: bool
                % True if we expanded some lambda expressions.
            ).

init_lambda_info(VarSet, VarTypes, TypeVarSet, InstVarSet, RttiVarMaps,
        HasParallelConj, PredInfo, ModuleInfo, Info) :-
    MustRecomputeNonLocals = no,
    HaveExpandedLambdas = no,
    Info = lambda_info(VarSet, VarTypes, TypeVarSet, InstVarSet,
        RttiVarMaps, HasParallelConj, PredInfo, ModuleInfo,
        MustRecomputeNonLocals, HaveExpandedLambdas).

lambda_info_get_varset(Info, Info ^ li_varset).
lambda_info_get_vartypes(Info, Info ^ li_vartypes).
lambda_info_get_tvarset(Info, Info ^ li_tvarset).
lambda_info_get_rtti_varmaps(Info, Info ^ li_rtti_varmaps).
lambda_info_get_inst_varset(Info, Info ^ li_inst_varset).
lambda_info_get_pred_info(Info, Info ^ li_pred_info).
lambda_info_get_module_info(Info, Info ^ li_module_info).
lambda_info_get_recompute_nonlocals(Info, Info ^ li_recompute_nonlocals).

lambda_info_set_varset(VarSet, !Info) :-
    !Info ^ li_varset := VarSet.
lambda_info_set_vartypes(VarTypes, !Info) :-
    !Info ^ li_vartypes := VarTypes.
lambda_info_set_module_info(ModuleInfo, !Info) :-
    !Info ^ li_module_info := ModuleInfo.
lambda_info_set_recompute_nonlocals(Recompute, !Info) :-
    !Info ^ li_recompute_nonlocals := Recompute.

%-----------------------------------------------------------------------------%
%
% This whole section just traverses the module structure.
%

expand_lambdas_in_module(!ModuleInfo) :-
    module_info_get_valid_pred_ids(!.ModuleInfo, PredIds),
    list.foldl(expand_lambdas_in_pred, PredIds, !ModuleInfo),
    % Need update the dependency graph to include the lambda predicates.
    module_info_clobber_dependency_info(!ModuleInfo).

expand_lambdas_in_pred(PredId, !ModuleInfo) :-
    module_info_pred_info(!.ModuleInfo, PredId, PredInfo),
    ProcIds = pred_info_procids(PredInfo),
    list.foldl(expand_lambdas_in_proc(PredId), ProcIds, !ModuleInfo).

:- pred expand_lambdas_in_proc(pred_id::in, proc_id::in,
    module_info::in, module_info::out) is det.

expand_lambdas_in_proc(PredId, ProcId, !ModuleInfo) :-
    module_info_get_preds(!.ModuleInfo, PredTable0),
    map.lookup(PredTable0, PredId, PredInfo0),
    pred_info_get_proc_table(PredInfo0, ProcTable0),
    map.lookup(ProcTable0, ProcId, ProcInfo0),

    expand_lambdas_in_proc_2(ProcInfo0, ProcInfo, PredInfo0, PredInfo1,
        !ModuleInfo),

    pred_info_get_proc_table(PredInfo1, ProcTable1),
    map.det_update(ProcId, ProcInfo, ProcTable1, ProcTable),
    pred_info_set_proc_table(ProcTable, PredInfo1, PredInfo),
    module_info_get_preds(!.ModuleInfo, PredTable1),
    map.det_update(PredId, PredInfo, PredTable1, PredTable),
    module_info_set_preds(PredTable, !ModuleInfo).

:- pred expand_lambdas_in_proc_2(proc_info::in, proc_info::out,
    pred_info::in, pred_info::out, module_info::in, module_info::out) is det.

expand_lambdas_in_proc_2(!ProcInfo, !PredInfo, !ModuleInfo) :-
    % Grab the appropriate fields from the pred_info and proc_info.
    pred_info_get_typevarset(!.PredInfo, TypeVarSet0),
    proc_info_get_headvars(!.ProcInfo, HeadVars),
    proc_info_get_varset(!.ProcInfo, VarSet0),
    proc_info_get_vartypes(!.ProcInfo, VarTypes0),
    proc_info_get_goal(!.ProcInfo, Goal0),
    proc_info_get_rtti_varmaps(!.ProcInfo, RttiVarMaps0),
    proc_info_get_inst_varset(!.ProcInfo, InstVarSet0),
    proc_info_get_has_parallel_conj(!.ProcInfo, HasParallelConj),
    MustRecomputeNonLocals0 = no,
    HaveExpandedLambdas0 = no,

    % Process the goal.
    Info0 = lambda_info(VarSet0, VarTypes0, TypeVarSet0, InstVarSet0,
        RttiVarMaps0, HasParallelConj, !.PredInfo, !.ModuleInfo,
        MustRecomputeNonLocals0, HaveExpandedLambdas0),
    expand_lambdas_in_goal(Goal0, Goal1, Info0, Info1),
    Info1 = lambda_info(VarSet1, VarTypes1, TypeVarSet, _InstVarSet,
        RttiVarMaps1, _, _PredInfo, !:ModuleInfo, MustRecomputeNonLocals,
        HaveExpandedLambdas),

    % Check if we need to requantify.
    (
        MustRecomputeNonLocals = yes,
        implicitly_quantify_clause_body_general(
            ordinary_nonlocals_no_lambda, HeadVars, _Warnings,
            Goal1, Goal2, VarSet1, VarSet2, VarTypes1, VarTypes2,
            RttiVarMaps1, RttiVarMaps2),
        proc_info_get_initial_instmap(!.ProcInfo, !.ModuleInfo, InstMap0),
        recompute_instmap_delta(recompute_atomic_instmap_deltas,
            Goal2, Goal, VarTypes2, InstVarSet0, InstMap0, !ModuleInfo)
    ;
        MustRecomputeNonLocals = no,
        Goal = Goal1,
        VarSet2 = VarSet1,
        VarTypes2 = VarTypes1,
        RttiVarMaps2 = RttiVarMaps1
    ),
    (
        HaveExpandedLambdas = yes,
        restrict_var_maps(HeadVars, Goal, VarSet2, VarSet, VarTypes2, VarTypes,
            RttiVarMaps2, RttiVarMaps)
    ;
        HaveExpandedLambdas = no,
        VarSet = VarSet2,
        VarTypes = VarTypes2,
        RttiVarMaps = RttiVarMaps2
    ),

    % Set the new values of the fields in proc_info and pred_info.
    proc_info_set_goal(Goal, !ProcInfo),
    proc_info_set_varset(VarSet, !ProcInfo),
    proc_info_set_vartypes(VarTypes, !ProcInfo),
    proc_info_set_rtti_varmaps(RttiVarMaps, !ProcInfo),
    pred_info_set_typevarset(TypeVarSet, !PredInfo).

:- pred expand_lambdas_in_goal(hlds_goal::in, hlds_goal::out,
    lambda_info::in, lambda_info::out) is det.

expand_lambdas_in_goal(Goal0, Goal, !Info) :-
    Goal0 = hlds_goal(GoalExpr0, GoalInfo),
    (
        GoalExpr0 = unify(LHS, RHS, Mode, Unification, Context),
        expand_lambdas_in_unify_goal(LHS, RHS, Mode, Unification, Context,
            GoalExpr, !Info)
    ;
        GoalExpr0 = conj(ConjType, Goals0),
        expand_lambdas_in_goal_list(Goals0, Goals, !Info),
        GoalExpr = conj(ConjType, Goals)
    ;
        GoalExpr0 = disj(Goals0),
        expand_lambdas_in_goal_list(Goals0, Goals, !Info),
        GoalExpr = disj(Goals)
    ;
        GoalExpr0 = switch(Var, CanFail, Cases0),
        expand_lambdas_in_cases(Cases0, Cases, !Info),
        GoalExpr = switch(Var, CanFail, Cases)
    ;
        GoalExpr0 = negation(SubGoal0),
        expand_lambdas_in_goal(SubGoal0, SubGoal, !Info),
        GoalExpr = negation(SubGoal)
    ;
        GoalExpr0 = scope(Reason, SubGoal0),
        (
            Reason = from_ground_term(_, FGT),
            ( FGT = from_ground_term_construct
            ; FGT = from_ground_term_deconstruct
            )
        ->
            % If the scope had any rhs_lambda_goals, modes.m wouldn't have
            % left its kind field as from_ground_term_(de)construct.
            GoalExpr = GoalExpr0
        ;
            expand_lambdas_in_goal(SubGoal0, SubGoal, !Info),
            GoalExpr = scope(Reason, SubGoal)
        )
    ;
        GoalExpr0 = if_then_else(Vars, Cond0, Then0, Else0),
        expand_lambdas_in_goal(Cond0, Cond, !Info),
        expand_lambdas_in_goal(Then0, Then, !Info),
        expand_lambdas_in_goal(Else0, Else, !Info),
        GoalExpr = if_then_else(Vars, Cond, Then, Else)
    ;
        ( GoalExpr0 = generic_call(_, _, _, _, _)
        ; GoalExpr0 = plain_call(_, _, _, _, _, _)
        ; GoalExpr0 = call_foreign_proc(_, _, _, _, _, _, _)
        ),
        GoalExpr = GoalExpr0
    ;
        GoalExpr0 = shorthand(ShortHand0),
        (
            ShortHand0 = atomic_goal(GoalType, Outer, Inner, MaybeOutputVars,
                MainGoal0, OrElseGoals0, OrElseInners),
            expand_lambdas_in_goal(MainGoal0, MainGoal, !Info),
            expand_lambdas_in_goal_list(OrElseGoals0, OrElseGoals, !Info),
            ShortHand = atomic_goal(GoalType, Outer, Inner, MaybeOutputVars,
                MainGoal, OrElseGoals, OrElseInners)
        ;
            ShortHand0 = try_goal(MaybeIO, ResultVar, SubGoal0),
            expand_lambdas_in_goal(SubGoal0, SubGoal, !Info),
            ShortHand = try_goal(MaybeIO, ResultVar, SubGoal)
        ;
            ShortHand0 = bi_implication(_, _),
            % These should have been expanded out by now.
            unexpected($module, $pred, "bi_implication")
        ),
        GoalExpr = shorthand(ShortHand)
    ),
    Goal = hlds_goal(GoalExpr, GoalInfo).

:- pred expand_lambdas_in_goal_list(list(hlds_goal)::in, list(hlds_goal)::out,
    lambda_info::in, lambda_info::out) is det.

expand_lambdas_in_goal_list([], [], !Info).
expand_lambdas_in_goal_list([Goal0 | Goals0], [Goal | Goals], !Info) :-
    expand_lambdas_in_goal(Goal0, Goal, !Info),
    expand_lambdas_in_goal_list(Goals0, Goals, !Info).

:- pred expand_lambdas_in_cases(list(case)::in, list(case)::out,
    lambda_info::in, lambda_info::out) is det.

expand_lambdas_in_cases([], [], !Info).
expand_lambdas_in_cases([Case0 | Cases0], [Case | Cases], !Info) :-
    Case0 = case(MainConsId, OtherConsIds, Goal0),
    expand_lambdas_in_goal(Goal0, Goal, !Info),
    Case = case(MainConsId, OtherConsIds, Goal),
    expand_lambdas_in_cases(Cases0, Cases, !Info).

:- pred expand_lambdas_in_unify_goal(prog_var::in, unify_rhs::in,
    unify_mode::in, unification::in, unify_context::in, hlds_goal_expr::out,
    lambda_info::in, lambda_info::out) is det.

expand_lambdas_in_unify_goal(LHS, RHS0, Mode, Unification0, Context, GoalExpr,
        !Info) :-
    (
        RHS0 = rhs_lambda_goal(Purity, Groundness, PredOrFunc, EvalMethod,
            NonLocalVars, Vars, Modes, Det, LambdaGoal0),
        % First, process the lambda goal recursively, in case it contains
        % some nested lambda expressions.
        expand_lambdas_in_goal(LambdaGoal0, LambdaGoal, !Info),

        % Then, convert the lambda expression into a new predicate.
        expand_lambda(Purity, Groundness, PredOrFunc, EvalMethod,
            not_reg_wrapper_proc, Vars, Modes, Det, NonLocalVars, LambdaGoal,
            Unification0, Y, Unification, !Info),
        GoalExpr = unify(LHS, Y, Mode, Unification, Context)
    ;
        ( RHS0 = rhs_var(_)
        ; RHS0 = rhs_functor(_, _, _)
        ),
        % We leave ordinary unifications unchanged.
        GoalExpr = unify(LHS, RHS0, Mode, Unification0, Context)
    ).

%-----------------------------------------------------------------------------%

expand_lambda(Purity, _Groundness, PredOrFunc, EvalMethod, RegWrapperProc,
        Vars, Modes, Detism, OrigNonLocals0, LambdaGoal, Unification0,
        Functor, Unification, LambdaInfo0, LambdaInfo) :-
    LambdaInfo0 = lambda_info(VarSet, VarTypes, TVarSet,
        InstVarSet, RttiVarMaps, HasParallelConj, OrigPredInfo,
        ModuleInfo0, MustRecomputeNonLocals0, _HaveExpandedLambdas),

    % Calculate the constraints which apply to this lambda expression.
    % Note currently we only allow lambda expressions to have universally
    % quantified constraints.
    rtti_varmaps_reusable_constraints(RttiVarMaps, AllConstraints),
    lookup_var_types(VarTypes, Vars, LambdaVarTypeList),
    list.map(type_vars, LambdaVarTypeList, LambdaTypeVarsList),
    list.condense(LambdaTypeVarsList, LambdaTypeVars),
    list.filter(constraint_contains_vars(LambdaTypeVars),
        AllConstraints, UnivConstraints),
    Constraints = constraints(UnivConstraints, []),

    % Existentially typed lambda expressions are not yet supported
    % (see the documentation at top of this file).
    ExistQVars = [],
    LambdaGoal = hlds_goal(_, LambdaGoalInfo),
    LambdaGoalNonLocals = goal_info_get_nonlocals(LambdaGoalInfo),
    set_of_var.insert_list(Vars, LambdaGoalNonLocals, LambdaNonLocals),
    goal_util.extra_nonlocal_typeinfos(RttiVarMaps, VarTypes, ExistQVars,
        LambdaNonLocals, ExtraTypeInfos),

    (
        Unification0 = construct(Var, _, OrigNonLocals1, UniModes0, _, _, _),
        % We used to use OrigVars = OrigNonLocals0 (from rhs_lambda_goal) but
        % the order of the variables does not necessarily match UniModes0.
        OrigVars = OrigNonLocals1,
        trace [compiletime(flag("lambda_var_order"))] (
            list.sort(OrigNonLocals0, SortedOrigNonLocals0),
            list.sort(OrigNonLocals1, SortedOrigNonLocals1),
            expect(unify(SortedOrigNonLocals0, SortedOrigNonLocals1),
                $module, $pred, "OrigNonLocals0 != OrigNonLocals1")
        )
    ;
        ( Unification0 = deconstruct(_, _, _, _, _, _)
        ; Unification0 = assign(_, _)
        ; Unification0 = simple_test(_, _)
        ; Unification0 = complicated_unify(_, _, _)
        ),
        unexpected($module, $pred, "unexpected unification")
    ),

    set_of_var.delete_list(Vars, LambdaGoalNonLocals, NonLocals1),

    % We need all the typeinfos, including the ones that are not used,
    % for the layout structure describing the closure.
    set_of_var.difference(ExtraTypeInfos, NonLocals1, NewTypeInfos),
    set_of_var.union(NonLocals1, NewTypeInfos, NonLocals),

    ( set_of_var.is_empty(NewTypeInfos) ->
        MustRecomputeNonLocals = MustRecomputeNonLocals0
    ;
        % If we added variables to the nonlocals of the lambda goal, then
        % we must recompute the nonlocals for the procedure that contains it.
        MustRecomputeNonLocals = yes
    ),

    set_of_var.to_sorted_list(NonLocals, ArgVars1),

    (
        % Optimize a special case: replace
        %   `(pred(Y1, Y2, ...) is Detism :-
        %       p(X1, X2, ..., Y1, Y2, ...))'
        % where `p' has determinism `Detism' with
        %   `p(X1, X2, ...)'
        %
        % This optimization is only valid if the modes of the Xi are input,
        % since only input arguments can be curried. It's also only valid
        % if all the inputs in the Yi precede the outputs. It's also not valid
        % if any of the Xi are in the Yi.

        LambdaGoal = hlds_goal(LambdaGoalExpr, _),
        LambdaGoalExpr = plain_call(PredId0, ProcId0, CallVars, _, _, _),
        module_info_pred_proc_info(ModuleInfo0, PredId0, ProcId0,
            Call_PredInfo, Call_ProcInfo),
        list.remove_suffix(CallVars, Vars, InitialVars),

        % Check that none of the variables that we're trying to use
        % as curried arguments are lambda-bound variables.
        \+ (
            list.member(InitialVar, InitialVars),
            list.member(InitialVar, Vars)
        ),

        % Check that the code models are compatible. Note that det is not
        % compatible with semidet, and semidet is not compatible with nondet,
        % since the calling conventions are different. If we're using the LLDS
        % back-end (i.e. not --high-level-code), det is compatible with nondet.
        % If we're using the MLDS back-end, then predicates and functions have
        % different calling conventions.
        Call_CodeModel = proc_info_interface_code_model(Call_ProcInfo),
        determinism_to_code_model(Detism, CodeModel),
        module_info_get_globals(ModuleInfo0, Globals),
        globals.get_target(Globals, Target),
        globals.lookup_bool_option(Globals, highlevel_code, HighLevelCode),
        (
            ( Target = target_c
            ; Target = target_csharp
            ; Target = target_java
            ),
            (
                HighLevelCode = no,
                (
                    CodeModel = Call_CodeModel
                ;
                    CodeModel = model_non,
                    Call_CodeModel = model_det
                )
            ;
                HighLevelCode = yes,
                Call_PredOrFunc = pred_info_is_pred_or_func(Call_PredInfo),
                PredOrFunc = Call_PredOrFunc,
                CodeModel = Call_CodeModel
            )
        ;
            Target = target_erlang,
            CodeModel = Call_CodeModel
        ),

        % Check that the curried arguments are all input.
        proc_info_get_argmodes(Call_ProcInfo, Call_ArgModes),
        list.length(InitialVars, NumInitialVars),
        list.take(NumInitialVars, Call_ArgModes, CurriedArgModes),
        (
            list.member(Mode, CurriedArgModes)
        =>
            mode_is_input(ModuleInfo0, Mode)
        )
    ->
        ArgVars = InitialVars,
        PredId = PredId0,
        ProcId = ProcId0,
        mode_util.modes_to_uni_modes(ModuleInfo0,
            CurriedArgModes, CurriedArgModes, UniModes),
        % We must mark the procedure as having had its address taken.
        proc_info_set_address_taken(address_is_taken,
            Call_ProcInfo, Call_NewProcInfo),
        module_info_set_pred_proc_info(PredId, ProcId,
            Call_PredInfo, Call_NewProcInfo, ModuleInfo0, ModuleInfo)
    ;
        % Prepare to create a new predicate for the lambda expression:
        % work out the arguments, module name, predicate name, arity,
        % arg types, determinism, context, status, etc. for the new predicate.

        ArgVars = put_typeinfo_vars_first(ArgVars1, VarTypes),
        list.append(ArgVars, Vars, AllArgVars),

        module_info_get_name(ModuleInfo0, ModuleName),
        OrigPredName = pred_info_name(OrigPredInfo),
        OrigContext = goal_info_get_context(LambdaGoalInfo),
        term.context_file(OrigContext, OrigFile),
        term.context_line(OrigContext, OrigLine),
        module_info_next_lambda_count(OrigContext, LambdaCount,
            ModuleInfo0, ModuleInfo1),
        make_pred_name_with_context(ModuleName, "IntroducedFrom",
            PredOrFunc, OrigPredName, OrigLine, LambdaCount, PredName),
        LambdaContext = goal_info_get_context(LambdaGoalInfo),
        % The TVarSet is a superset of what it really ought be,
        % but that shouldn't matter.
        % Existentially typed lambda expressions are not yet supported
        % (see the documentation at top of this file).
        ExistQVars = [],
        uni_modes_to_modes(UniModes0, OrigArgModes),

        % We have to jump through hoops to work out the mode of the lambda
        % predicate. For introduced type_info arguments, we use the mode "in".
        % For the original non-local vars, we use the modes from `UniModes1'.
        % For the lambda var arguments at the end, we use the mode in the
        % lambda expression.

        list.length(ArgVars, NumArgVars),
        in_mode(In),
        list.duplicate(NumArgVars, In, InModes),
        map.from_corresponding_lists(ArgVars, InModes, ArgModesMap),

        map.from_corresponding_lists(OrigVars, OrigArgModes, OrigArgModesMap),
        map.overlay(ArgModesMap, OrigArgModesMap, ArgModesMap1),
        map.apply_to_list(ArgVars, ArgModesMap1, ArgModes1),

        % Recompute the uni_modes.
        modes_to_uni_modes(ModuleInfo1, ArgModes1, ArgModes1, UniModes),

        list.append(ArgModes1, Modes, AllArgModes),
        lookup_var_types(VarTypes, AllArgVars, ArgTypes),
        list.foldl_corresponding(check_lambda_arg_type_and_mode(ModuleInfo1),
            ArgTypes, AllArgModes, 0, _),

        purity_to_markers(Purity, PurityMarkers),
        init_markers(LambdaMarkers0),
        add_markers(PurityMarkers, LambdaMarkers0, LambdaMarkers),

        % Now construct the proc_info and pred_info for the new single-mode
        % predicate, using the information computed above.
        map.init(VarNameRemap),
        restrict_var_maps(AllArgVars, LambdaGoal, VarSet, LambdaVarSet,
            VarTypes, LambdaVarTypes, RttiVarMaps, LambdaRttiVarMaps),
        some [!ProcInfo] (
            % If the original procedure contained parallel conjunctions,
            % then the one we are creating here may have them as well.
            % If it does not, then the value in the proc_info of the lambda
            % predicate will be an overconservative estimate.
            proc_info_create(LambdaContext, LambdaVarSet, LambdaVarTypes,
                AllArgVars, InstVarSet, AllArgModes, detism_decl_explicit,
                Detism, LambdaGoal, LambdaRttiVarMaps, address_is_taken,
                HasParallelConj, VarNameRemap, !:ProcInfo),

            % The debugger ignores unnamed variables.
            ensure_all_headvars_are_named(!ProcInfo),

            % If we previously already needed to recompute the nonlocals,
            % then we had better apply that recomputation for the procedure
            % that we just created.
            (
                MustRecomputeNonLocals0 = yes,
                requantify_proc_general(ordinary_nonlocals_maybe_lambda,
                    !ProcInfo)
            ;
                MustRecomputeNonLocals0 = no
            ),
            (
                RegWrapperProc = reg_wrapper_proc(RegR_HeadVars),
                proc_info_set_reg_r_headvars(RegR_HeadVars, !ProcInfo)
            ;
                RegWrapperProc = not_reg_wrapper_proc
            ),
            ProcInfo = !.ProcInfo
        ),
        set.init(Assertions),
        pred_info_create(ModuleName, PredName, PredOrFunc, LambdaContext,
            origin_lambda(OrigFile, OrigLine, LambdaCount),
            pred_status(status_local), LambdaMarkers, ArgTypes, TVarSet,
            ExistQVars, Constraints, Assertions, VarNameRemap,
            ProcInfo, ProcId, PredInfo),

        % Save the new predicate in the predicate table.
        module_info_get_predicate_table(ModuleInfo1, PredicateTable0),
        predicate_table_insert(PredInfo, PredId,
            PredicateTable0, PredicateTable),
        module_info_set_predicate_table(PredicateTable,
            ModuleInfo1, ModuleInfo)
    ),
    ShroudedPredProcId = shroud_pred_proc_id(proc(PredId, ProcId)),
    ConsId = closure_cons(ShroudedPredProcId, EvalMethod),
    Functor = rhs_functor(ConsId, is_not_exist_constr, ArgVars),

    Unification = construct(Var, ConsId, ArgVars, UniModes,
        construct_dynamically, cell_is_unique, no_construct_sub_info),
    HaveExpandedLambdas = yes,
    LambdaInfo = lambda_info(VarSet, VarTypes, TVarSet,
        InstVarSet, RttiVarMaps, HasParallelConj, OrigPredInfo,
        ModuleInfo, MustRecomputeNonLocals, HaveExpandedLambdas).

:- pred constraint_contains_vars(list(tvar)::in, prog_constraint::in)
    is semidet.

constraint_contains_vars(LambdaVars, ClassConstraint) :-
    ClassConstraint = constraint(_, ConstraintTypes),
    list.map(type_vars, ConstraintTypes, ConstraintVarsList),
    list.condense(ConstraintVarsList, ConstraintVars),
    % Probably not the most efficient way of doing it, but I wouldn't think
    % that it matters.
    set.list_to_set(LambdaVars, LambdaVarsSet),
    set.list_to_set(ConstraintVars, ConstraintVarsSet),
    set.subset(ConstraintVarsSet, LambdaVarsSet).

    % This predicate works out the modes of the original non-local variables
    % of a lambda expression based on the list of uni_mode in the unify_info
    % for the lambda unification.
    %
:- pred uni_modes_to_modes(list(uni_mode)::in, list(mer_mode)::out) is det.

uni_modes_to_modes([], []).
uni_modes_to_modes([UniMode | UniModes], [Mode | Modes]) :-
    UniMode = ((_Initial0 - Initial1) -> (_Final0 - _Final1)),
    Mode = (Initial1 -> Initial1),
    uni_modes_to_modes(UniModes, Modes).

    % Make sure the arguments and modes are not misordered. An obvious
    % indicator is if a non-higher order argument is paired a higher order
    % inst.
    %
:- pred check_lambda_arg_type_and_mode(module_info::in, mer_type::in,
    mer_mode::in, int::in, int::out) is det.

check_lambda_arg_type_and_mode(ModuleInfo, Type, Mode, X, X) :-
    Inst = mode_get_initial_inst(ModuleInfo, Mode),
    ( Inst = ground(_, higher_order(_)) ->
        ( type_is_higher_order(Type) ->
            true
        ;
            unexpected($module, $pred,
                "non-higher order argument with higher order inst")
        )
    ;
        true
    ).

%---------------------------------------------------------------------------%

    % The proc_info has several maps that refer to variables. After lambda
    % expansion, both the newly created procedures and the original procedure
    % that they were carved out of have duplicate copies of these maps.
    % This duplication is a problem because later passes (in particular,
    % the equiv_types_hlds pass) iterate over the entries in these maps,
    % and if an entry is duplicated N times, they have to process it N times.
    % The task of this predicate is to eliminate unnecessary entries
    % from the vartypes map, and this requires also eliminating them from
    % the rtti_varmaps.
    %
    % We could in theory restrict the varsets in the proc_info as well
    % both the main prog_varset and the other varsets, e.g. the tvarset),
    % but since we don't iterate over those sets, there is (as yet) no need
    % for this.
    %
:- pred restrict_var_maps(list(prog_var)::in, hlds_goal::in,
    prog_varset::in, prog_varset::out, vartypes::in, vartypes::out,
    rtti_varmaps::in, rtti_varmaps::out) is det.

restrict_var_maps(HeadVars, Goal, !VarSet, !VarTypes, !RttiVarMaps) :-
    MaxVar = varset.max_var(!.VarSet),
    MaxVarNum = var_to_int(MaxVar),
    % Variable numbers go from 1 to MaxVarNum. Reserve array slots
    % from 0 to MaxVarNum, since wasting the space of one array element
    % is preferable to having to always to do a subtraction on every array
    % lookup.
    array.init(MaxVarNum + 1, no, VarUses0),
    mark_vars_as_used(HeadVars, VarUses0, VarUses1),
    find_used_vars_in_goal(Goal, VarUses1, VarUses),

    vartypes_to_assoc_list(!.VarTypes, VarTypesList0),
    filter_vartypes(VarTypesList0, [], RevVarTypesList, VarUses),
    list.reverse(RevVarTypesList, VarTypesList),
    vartypes_from_sorted_assoc_list(VarTypesList, !:VarTypes),

    restrict_rtti_varmaps(VarUses, !RttiVarMaps).

:- pred filter_vartypes(assoc_list(prog_var, mer_type)::in,
    assoc_list(prog_var, mer_type)::in, assoc_list(prog_var, mer_type)::out,
    array(bool)::in) is det.

filter_vartypes([], !RevVarTypes, _VarUses).
filter_vartypes([VarType | VarTypes], !RevVarTypes, VarUses) :-
    VarType = Var - _Type,
    VarNum = var_to_int(Var),
    array.unsafe_lookup(VarUses, VarNum, Used),
    (
        Used = yes,
        !:RevVarTypes = [VarType | !.RevVarTypes]
    ;
        Used = no
    ),
    filter_vartypes(VarTypes, !RevVarTypes, VarUses).

:- pred find_used_vars_in_goal(hlds_goal::in,
    array(bool)::array_di, array(bool)::array_uo) is det.

find_used_vars_in_goal(Goal, !VarUses) :-
    Goal = hlds_goal(GoalExpr, _GoalInfo),
    (
        GoalExpr = unify(LHSVar, RHS, _, Unif, _),
        mark_var_as_used(LHSVar, !VarUses),
        (
            Unif = construct(_, _, _, _, CellToReuse, _, _),
            ( CellToReuse = reuse_cell(cell_to_reuse(ReuseVar, _, _)) ->
                mark_var_as_used(ReuseVar, !VarUses)
            ;
                true
            )
        ;
            Unif = deconstruct(_, _, _, _, _, _)
        ;
            Unif = assign(_, _)
        ;
            Unif = simple_test(_, _)
        ;
            Unif = complicated_unify(_, _, _)
        ),
        (
            RHS = rhs_var(RHSVar),
            mark_var_as_used(RHSVar, !VarUses)
        ;
            RHS = rhs_functor(_, _, ArgVars),
            mark_vars_as_used(ArgVars, !VarUses)
        ;
            RHS = rhs_lambda_goal(_, _, _, _, NonLocals, LambdaVars,
                _, _, LambdaGoal),
            mark_vars_as_used(NonLocals, !VarUses),
            mark_vars_as_used(LambdaVars, !VarUses),
            find_used_vars_in_goal(LambdaGoal, !VarUses)
        )
    ;
        GoalExpr = generic_call(GenericCall, ArgVars, _, _, _),
        (
            GenericCall = higher_order(Var, _, _, _),
            mark_var_as_used(Var, !VarUses)
        ;
            GenericCall = class_method(Var, _, _, _),
            mark_var_as_used(Var, !VarUses)
        ;
            GenericCall = event_call(_)
        ;
            GenericCall = cast(_)
        ),
        mark_vars_as_used(ArgVars, !VarUses)
    ;
        GoalExpr = plain_call(_, _, ArgVars, _, _, _),
        mark_vars_as_used(ArgVars, !VarUses)
    ;
        ( GoalExpr = conj(_, Goals)
        ; GoalExpr = disj(Goals)
        ),
        find_used_vars_in_goals(Goals, !VarUses)
    ;
        GoalExpr = switch(Var, _Det, Cases),
        mark_var_as_used(Var, !VarUses),
        find_used_vars_in_cases(Cases, !VarUses)
    ;
        GoalExpr = scope(Reason, SubGoal),
        (
            Reason = exist_quant(Vars),
            mark_vars_as_used(Vars, !VarUses)
        ;
            Reason = promise_solutions(Vars, _),
            mark_vars_as_used(Vars, !VarUses)
        ;
            Reason = from_ground_term(Var, _),
            mark_var_as_used(Var, !VarUses)
        ;
            Reason = loop_control(LCVar, LCSVar, _),
            mark_var_as_used(LCVar, !VarUses),
            mark_var_as_used(LCSVar, !VarUses)
        ;
            ( Reason = promise_purity(_)
            ; Reason = barrier(_)
            ; Reason = commit(_)
            ; Reason = trace_goal(_, _, _, _, _)
            )
            % Do nothing.
        ;
            ( Reason = require_detism(_)
            ; Reason = require_complete_switch(_)
            ; Reason = require_switch_arms_detism(_, _)
            ),
            % These scopes should have been deleted by now.
            unexpected($module, $pred, "unexpected scope")
        ),
        find_used_vars_in_goal(SubGoal, !VarUses)
    ;
        GoalExpr = negation(SubGoal),
        find_used_vars_in_goal(SubGoal, !VarUses)
    ;
        GoalExpr = if_then_else(Vars, Cond, Then, Else),
        mark_vars_as_used(Vars, !VarUses),
        find_used_vars_in_goal(Cond, !VarUses),
        find_used_vars_in_goal(Then, !VarUses),
        find_used_vars_in_goal(Else, !VarUses)
    ;
        GoalExpr = call_foreign_proc(_, _, _, Args, ExtraArgs, _, _),
        ArgVars = list.map(foreign_arg_var, Args),
        ExtraVars = list.map(foreign_arg_var, ExtraArgs),
        mark_vars_as_used(ArgVars, !VarUses),
        mark_vars_as_used(ExtraVars, !VarUses)
    ;
        GoalExpr = shorthand(Shorthand),
        (
            Shorthand = atomic_goal(_, Outer, Inner, MaybeOutputVars,
                MainGoal, OrElseGoals, _),
            Outer = atomic_interface_vars(OuterDI, OuterUO),
            mark_var_as_used(OuterDI, !VarUses),
            mark_var_as_used(OuterUO, !VarUses),
            Inner = atomic_interface_vars(InnerDI, InnerUO),
            mark_var_as_used(InnerDI, !VarUses),
            mark_var_as_used(InnerUO, !VarUses),
            (
                MaybeOutputVars = no
            ;
                MaybeOutputVars = yes(OutputVars),
                mark_vars_as_used(OutputVars, !VarUses)
            ),
            find_used_vars_in_goal(MainGoal, !VarUses),
            find_used_vars_in_goals(OrElseGoals, !VarUses)
        ;
            Shorthand = try_goal(_, _, SubGoal),
            % The IO and Result variables would be in SubGoal.
            find_used_vars_in_goal(SubGoal, !VarUses)
        ;
            Shorthand = bi_implication(LeftGoal, RightGoal),
            find_used_vars_in_goal(LeftGoal, !VarUses),
            find_used_vars_in_goal(RightGoal, !VarUses)
        )
    ).

:- pred find_used_vars_in_goals(list(hlds_goal)::in,
    array(bool)::array_di, array(bool)::array_uo) is det.

find_used_vars_in_goals([], !VarUses).
find_used_vars_in_goals([Goal | Goals], !VarUses) :-
    find_used_vars_in_goal(Goal, !VarUses),
    find_used_vars_in_goals(Goals, !VarUses).

:- pred find_used_vars_in_cases(list(case)::in,
    array(bool)::array_di, array(bool)::array_uo) is det.

find_used_vars_in_cases([], !VarUses).
find_used_vars_in_cases([Case | Cases], !VarUses) :-
    Case = case(_, _, Goal),
    find_used_vars_in_goal(Goal, !VarUses),
    find_used_vars_in_cases(Cases, !VarUses).

:- pred mark_var_as_used(prog_var::in,
    array(bool)::array_di, array(bool)::array_uo) is det.
:- pragma inline(mark_var_as_used/3).

mark_var_as_used(Var, !VarUses) :-
    array.set(var_to_int(Var), yes, !VarUses).

:- pred mark_vars_as_used(list(prog_var)::in,
    array(bool)::array_di, array(bool)::array_uo) is det.

mark_vars_as_used([], !VarUses).
mark_vars_as_used([Var | Vars], !VarUses) :-
    mark_var_as_used(Var, !VarUses),
    mark_vars_as_used(Vars, !VarUses).

%---------------------------------------------------------------------------%
:- end_module transform_hlds.lambda.
%---------------------------------------------------------------------------%