I'm new to prolog, so this's quite a challenge to me. I'm supposed to implement a simple C like language in Prolog.
the ultimate goal is to be able to execute something like this:
?- run([begin,a,:=,10,while,a,>,5,begin,write,a,a,:=,a,-,1,end,end]).
and get:
10
9
8
7
6
yes
However, I'm stuck at the first step. This is what I have achieved so far. out of local stack!
statement(Vars,_Vars) --> assign(Vars,_Vars).
statement(Vars,Vars2) --> statement(Vars,Vars1), statement(Vars1,Vars2).
assign(Vars,_Vars) --> default_assign(Vars,_Vars).
assign(Vars,_Vars) --> simple_assign(Vars,_Vars).
% a //default value 0
default_assign(Vars,_Vars) -->
var_name(Var_Name),
{update_vars([Var_Name,0],Vars,_Vars)}.
% a = 0
simple_assign(Vars,_Vars) -->
var_name(Var_Name),[=],var_value(Var_Value),
{update_vars([Var_Name,Var_Value],Vars,_Vars)}.
% a = b
simple_assign(Vars,_Vars) -->
var_name(Var_Name1),[=],var_name(Var_Name2),
{
update_vars([Var_Name1,Var_Value],Vars,_Vars)
}.
var_name(Var_Name) --> [Var_Name],{\+number(Var_Name2)}.
var_value(Var_Value) -->[Var_Value],{number(Var_Value)}.
% found match, update
update_vars(Var,Vars,_Vars):-
member(Var,Vars),
update(Var,Vars,_Vars),
_Vars\==[].
% no match, append
update_vars(Var,Vars,_Vars):-
\+member(Var,Vars),
append(Var,Vars,_Vars).
update([Name,Value],[],[]).
update([Name,Value],[[Name,Old_Value]|T1],[[Name,Value]|T2]):-
update([Name,Value],T1,T2).
update([Name,Value],[[Name1,Value1]|T1],[[Name1,Value1]|T2]):-
[Name,Value]\=[Name1,Value1],
update([Name,Value],T1,T2).
append([Name,Value],[],[[Name,Value]]).
append([Name,Value],[H|T1],[H|T2]):-
append([Name,Value],T1,T2).
Here's my logic. First I want to be able to consume the list(that's how I interpret it - -!), so the grammar structure is really really important. And I'm also thinking about using a variable list 'Vars' in forms of [[Name,Value],[a,1],[b,2]...], and an updated version - '_Vars'. So I can pass it to other statements like while loop and write.
statement(Vars,Vars2) --> statement(Vars,Vars1), statement(Vars1,Vars2).
% this seems wrong...
But... It looks like the logic is wrong from the beginning. :\ below is the simplified version. I would really appreciate it if you can help me out here. And I really hope I won't take this with me in Christmas. T.T
statement --> assign.
statement --> statement, statement.
assign --> simple_assign.
assign --> default_assign.
default_assign -->
var_name(Var_Name).
simple_assign -->
var_name,[=],var_value.
var_name -->
[Var_Name],{\+number(Var_Name)}.
var_value -->
[Var_Value],{number(Var_Value)}.
This is how I would go about it:
transform the source code into a abstract syntax tree
begin
a := 1
while a < 5
begin
a := a + 1;
end
end
becomes
statements([
assign(a, number(1)),
while(greater(variable(a), number(5))),
statements([
assign(a, plus(variable(a), number(1)))
])
)
])
build an interpreter for it.
There are various interpreters. The easiest one is the vanilla interpreter. Here is one I would begin with:
interpret(number(N), State, N, State).
interpret(assign(Variable, Statement), State, Value, NewState) :-
interpret(Statement, State, Value, NewState1),
assignVariable(Variable, Value, NewState1, NewState).
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