:- module small.
:- interface.

:- import_module list.

:- parse(program/1, token, ('$'), xx, in, out).

:- rule program(list(defn)).
program(Defs) --->
    (
        definition(Def),
        { Defs = [Def] }
    ;
        program(Defs0), definition(Def),
        { append(Defs0, [Def], Defs) }
    ).

:- rule definition(defn).
definition(Def) --->
    (
        globals(Glob), { Def = glob(Glob) }
    ;
        function(Fun), { Def = fun(Fun) }
    ).

:- rule globals(pair((type), list(id))).
globals(Type - Vars) ---> type(Type), vars(Vars), [';'].

:- rule function(fun).
function(Fun) --->
    type(RType), [id(Name)], ['('], parameters(Params), [')'],
    compound(Stmnt),
    { Fun = fun(Name, RType, Params, Stmnt) }.

:- rule parameters(list(pair((type), id))).
parameters([])    --->  [].
parameters(PList) --->  parameter_list(PList).

:- rule parameter_list(list(pair((type), id))).
parameter_list(Params) --->
    (
        parameter(Param),
        { Params = [Param] }
    ;
        parameter(Param), [','], parameter_list(Params0),
        { Params = [Params0|Params] }
    ).

:- rule parameter(pair((type), id)).
parameter(Type - Id) ---> type(Type), var(Id).

:- rule compound(statement).
compound(compound(Statements)) --->
    ['{'], statements(Statements), ['}'].

:- rule statements(list(statement)).
statements(Statements) --->
    (
        statement(Statement),
        { Statements = [Statement] }
    ;
        statements(Statements0), statement(Statement),
        { Statements = [Statement|Statements0] }
    ).

:- rule statement(statement).
statement(Stmnt) --->
        compound(Stmnt)
    ;   ifthenelse(Stmnt)
    ;   while(Stmnt)
    ;   assignment(Stmnt)
    .

:- rule ifthenelse(statement).
ifthenelse(Stmnt) --->
    (
        ['if'], ['('], expression(Cond), [')'], statement(Then),
        { Stmnt = ite(Cond, Then, compound([])) }
    ;
        ['if'], ['('], expression(Cond), [')'],
        statement(Then), ['else'], statement(Else),
        { Stmnt = ite(Cond, Then, Else) }
    ).

:- rule while(statement).
while(Stmnt) --->
    ['while'], ['('], expression(Cond), [')'], statement(Body),
    { Stmnt = while(Cond, Body) }.

:- rule assignment(statement).
assignment(assign(Var, Expr)) --->
    var(Var), ['='], expression(Expr).

:- rule expression(expression).
expression(E) ---> expression(E0), ['+'], term(E1), { E = E0 + E1 }.
expression(E) ---> expression(E0), ['-'], term(E1), { E = E0 - E1 }.
expression(E) ---> term(E).

:- rule term(expression).
term(E) ---> term(E0), ['*'], factor(E1), { E = E0 * E1 }.
term(E) ---> term(E0), ['/'], factor(E1), { E = E0 / E1 }.
term(E) ---> factor(E).

:- rule factor(expression).
factor(E) --->
    (
        [id(Var)], { E = var(Var) }
    ;
        [num(Num)], { E = const(Num) }
    ;
        [id(Func)], ['('], vars(Args), [')'],
        { E = fun(Func, Args) }
    ;
        ['('], expression(E), [')']
    ).

:- rule vars(list(id)).
vars([])   ---> [].
vars(Vars) ---> vars1(Vars).

:- rule vars1(list(id)).
vars1(Vars) --->
    (
        var(Var),
        { Vars = [Var] }
    ;
        var(Var), [','], vars1(Vars0),
        { Vars = [Var|Vars0] }
    ).

:- rule var(id).
var(Id) ---> [id(Id)].

:- rule type(type).
type(Type) --->
    (
        [int],
        { Type = int }
    ;
        ['('], types(Types), [')'],
        { ( Types = [Type0] ->
            Type = Type0
          ;
            Type = tuple(Types)
        ) }
    ).

:- rule types(list(type)).
types(Types) --->
    (
        type(Type),
        { Types = [Type] }
    ;
        types(Types0), type(Type),
        { append(Types0, [Type], Types) }
    ).