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109 varset

%--------------------------------------------------%
% vim: ft=mercury ts=4 sw=4 et
%--------------------------------------------------%
% Copyright (C) 1993-2000,2002-2007, 2009-2011 The University of Melbourne.
% Copyright (C) 2014-2018 The Mercury team.
% This file is distributed under the terms specified in COPYING.LIB.
%--------------------------------------------------%
%
% File: varset.m.
% Main author: fjh.
% Stability: low.
%
% This file provides facilities for manipulating collections of variables.
% through the 'varset' ADT. These variables are object-level variables,
% and are represented as ground terms, so it might help to think of them
% as "variable ids" rather than Prolog-style variables.
%
% A varset may record a name and/or a value (binding) with each variable.
%
% Many situations require dealing with several distinct sets of variables
% that should never be mixed together. For example, a compiler may handle
% both program variables and type variables, and it does not make sense
% to a have a single varset containing both program variables and type
% variables. The varsets provided by this module are thus parameterized;
% the compiler can use e.g. varset(prog_var_type) to hold program variables
% and varset(tvar_type) to hold type variables. Since all operations on
% two or more varsets require agreement on the argument of the varset/1
% type constructor, any accidental mixup of different instances of varset/1
% is guaranteed to be caught by the compiler.
%
% In situations in which this is not a concern, programmers may use
% the standard generic varset instance.
%
% Note that there are some design flaws in the relationship between
% varset.m and term.m. There is too much coupling between the two,
% which may and should be fixed later, e.g. by merging the two modules.
%
%--------------------------------------------------%
%--------------------------------------------------%

:- module varset.
:- interface.

:- import_module assoc_list.
:- import_module list.
:- import_module map.
:- import_module maybe.
:- import_module set.
:- import_module term.

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

:- type varset(T).
:- type varset == varset(generic).

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

    % Construct an empty varset.
    %
:- func init = varset(T).
:- pred init(varset(T)::out) is det.

    % Check whether a varset is empty.
    %
:- pred is_empty(varset(T)::in) is semidet.

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

    % Create a new variable.
    %
:- pred new_var(var(T)::out, varset(T)::in, varset(T)::out) is det.

    % Create a new named variable.
    %
:- pred new_named_var(string::in, var(T)::out,
    varset(T)::in, varset(T)::out) is det.

    % Create a new named variable with a unique (w.r.t. the varset) number
    % appended to the name.
    %
:- pred new_uniquely_named_var(string::in, var(T)::out,
    varset(T)::in, varset(T)::out) is det.

    % Create a new variable, and maybe give it a name.
    %
:- pred new_maybe_named_var(maybe(string)::in, var(T)::out,
    varset(T)::in, varset(T)::out) is det.

    % Create multiple new variables.
    % Throws an exception if a negative number of new variables
    % is requested.
    %
:- pred new_vars(int::in, list(var(T))::out,
    varset(T)::in, varset(T)::out) is det.

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

    % Delete the name and value for a variable.
    %
:- func delete_var(varset(T), var(T)) = varset(T).
:- pred delete_var(var(T)::in, varset(T)::in, varset(T)::out) is det.

    % Delete the names and values for a list of variables.
    %
:- func delete_vars(varset(T), list(var(T))) = varset(T).
:- pred delete_vars(list(var(T))::in, varset(T)::in, varset(T)::out)
    is det.

    % Delete the names and values for a sorted list of variables.
    % (If the list is not sorted, the predicate or function will
    % either throw an exception or return incorrect output.)
    %
:- func delete_sorted_vars(varset(T), list(var(T))) = varset(T).
:- pred delete_sorted_vars(list(var(T))::in,
    varset(T)::in, varset(T)::out) is det.

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

    % Return a list of all the variables in a varset.
    %
:- func vars(varset(T)) = list(var(T)).
:- pred vars(varset(T)::in, list(var(T))::out) is det.

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

    % Set the name of a variable.
    %
:- func name_var(varset(T), var(T), string) = varset(T).
:- pred name_var(var(T)::in, string::in,
    varset(T)::in, varset(T)::out) is det.

    % Unset the name of a variable.
    %
:- func unname_var(varset(T), var(T)) = varset(T).
:- pred unname_var(var(T)::in, varset(T)::in, varset(T)::out) is det.

    % Lookup the name of a variable;
    % If it doesn't have one, return a default name consisting of two parts:
    % "V_" as a prefix, followed by a unique number. This is meant to evoke
    % "variable number N".
    %
:- func lookup_name(varset(T), var(T)) = string.
:- pred lookup_name(varset(T)::in, var(T)::in, string::out) is det.

    % Lookup the name of a variable;
    % if it doesn't have one, create one using the specified prefix.
    %
:- func lookup_name(varset(T), var(T), string) = string.
:- pred lookup_name(varset(T)::in, var(T)::in, string::in, string::out)
    is det.

    % Lookup the name of a variable;
    % fail if it doesn't have one.
    %
:- pred search_name(varset(T)::in, var(T)::in, string::out) is semidet.

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

    % Bind a value to a variable.
    % This will overwrite any existing binding.
    %
:- func bind_var(varset(T), var(T), term(T)) = varset(T).
:- pred bind_var(var(T)::in, term(T)::in,
    varset(T)::in, varset(T)::out) is det.

    % Bind a set of terms to a set of variables.
    %
:- func bind_vars(varset(T), substitution(T)) = varset(T).
:- pred bind_vars(substitution(T)::in,
    varset(T)::in, varset(T)::out) is det.

    % Lookup the value of a variable.
    %
:- pred search_var(varset(T)::in, var(T)::in, term(T)::out) is semidet.

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

    % Get the bindings for all the bound variables.
    %
:- func lookup_vars(varset(T)) = substitution(T).
:- pred lookup_vars(varset(T)::in, substitution(T)::out) is det.

    % Get the bindings for all the bound variables.
    %
:- func get_bindings(varset(T)) = substitution(T).
:- pred get_bindings(varset(T)::in, substitution(T)::out) is det.

    % Set the bindings for all the bound variables.
    %
:- func set_bindings(varset(T), substitution(T)) = varset(T).
:- pred set_bindings(varset(T)::in, substitution(T)::in,
    varset(T)::out) is det.

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

    % Combine two different varsets, renaming apart:
    % merge_renaming(VarSet0, NewVarSet, VarSet, Renaming) is true
    % iff VarSet is the varset that results from joining a suitably renamed
    % version of NewVarSet to VarSet0. (Any bindings in NewVarSet are ignored.)
    % Renaming will map each variable in NewVarSet to the corresponding
    % fresh variable in VarSet.
    %
:- pred merge_renaming(varset(T)::in, varset(T)::in, varset(T)::out,
    renaming(T)::out) is det.

    % Same as merge_renaming, except that the names of variables
    % in NewVarSet are not included in the final varset.
    % This is useful if create_name_var_map needs to be used
    % on the resulting varset.
    %
:- pred merge_renaming_without_names(varset(T)::in,
    varset(T)::in, varset(T)::out, renaming(T)::out) is det.

    % merge(VarSet0, NewVarSet, Terms0, VarSet, Terms):
    %
    % As merge_renaming, except instead of returning the renaming,
    % this predicate applies it to the given list of terms.
    %
:- pred merge(varset(T)::in, varset(T)::in, list(term(T))::in,
    varset(T)::out, list(term(T))::out) is det.

    % Same as merge, except that the names of variables
    % in NewVarSet are not included in the final varset.
    % This is useful if create_name_var_map needs to be used
    % on the resulting varset.
    %
:- pred merge_without_names(varset(T)::in, varset(T)::in,
    list(term(T))::in, varset(T)::out, list(term(T))::out) is det.

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

    % Create a map from names to variables.
    % Each name is mapped to only one variable, even if a name is
    % shared by more than one variable. Therefore this predicate
    % is only really useful if it is already known that no two
    % variables share the same name.
    %
:- func create_name_var_map(varset(T)) = map(string, var(T)).
:- pred create_name_var_map(varset(T)::in, map(string, var(T))::out)
    is det.

    % Return an association list giving the name of each variable.
    % Every variable has an entry in the returned association list,
    % even if it shares its name with another variable.
    %
:- func var_name_list(varset(T)) = assoc_list(var(T), string).
:- pred var_name_list(varset(T)::in, assoc_list(var(T), string)::out)
    is det.

    % Given a list of variable and varset in which some variables have
    % no name but some other variables may have the same name,
    % return another varset in which every variable has a unique name.
    % If necessary, names will have suffixes added on the end;
    % the second argument gives the suffix to use.
    %
:- func ensure_unique_names(list(var(T)), string, varset(T))
    = varset(T).
:- pred ensure_unique_names(list(var(T))::in,
    string::in, varset(T)::in, varset(T)::out) is det.

    % Unname all variables whose explicitly given names have the form
    % of the default names used by lookup_name, i.e. "V_" followed by
    % an integer.
    %
    % This predicate is intended to be used in situations where
    % a term has been read in after being written out. The process of
    % writing out the term forces requires every variable to given
    % a name that can be written out, even variables that until then
    % did not have names. If these variables are given names of the default
    % form, then, after the written-out term is read back in, this predicate
    % will recreate the original varset, including the variables without names.
    %
:- pred undo_default_names(varset(T)::in, varset(T)::out) is det.

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

    % Given a varset and a set of variables, remove the names
    % and values of any other variables stored in the varset.
    %
:- func select(varset(T), set(var(T))) = varset(T).
:- pred select(set(var(T))::in, varset(T)::in, varset(T)::out) is det.

    % Given a varset and a list of variables, construct a new varset
    % containing one variable for each one in the list (and no others).
    % Also return a substitution mapping the selected variables in the
    % original varset into variables in the new varset. The relative
    % ordering of variables in the original varset is maintained.
    %
:- pred squash(varset(T)::in, list(var(T))::in,
    varset(T)::out, renaming(T)::out) is det.

    % Coerce the types of the variables in a varset.
    %
:- func coerce(varset(T)) = varset(U).
:- pred coerce(varset(T)::in, varset(U)::out) is det.

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


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