public static void LambdaCalculus(string program, string parsed, string eval)
{
var term = LcTerm.Parse(program);
var env = System.Collections.Immutable.ImmutableDictionary.Create <string, LcTerm>();
Assert.Equal(parsed, term.ToString());
Assert.Equal(eval, term.Eval(env)?.ToString());
}
static bool Parse(Input inp, ref Position pos, ref Rules rules, out LcTerm x)
{
inp.WhiteSpaces(ref pos);
return(LcApply(inp, ref pos, ref rules, out x) ||
LcLambda(inp, ref pos, ref rules, out x) ||
LcBracketed(inp, ref pos, ref rules, out x) ||
LcVar(inp, ref pos, out x));
}
static bool LcVar(Input inp, ref Position pos, out LcTerm x)
{
if (inp.Letters(ref pos, out var name))
{
x = new LcVar {
Name = name.ToString(), Position = pos.Pos
};
return(true);
}
x = default;
return(false);
}
static bool LcBracketed(Input inp, ref Position pos, ref Rules rules, out LcTerm x)
{
var i = pos;
if (inp.Char('(', ref i) &&
Parse(inp, ref i, ref rules, out x) &&
inp.Optional(inp.WhiteSpaces(ref i)) &&
inp.Char(')', ref i))
{
return(pos.Seek(i, ref rules));
}
return(Recognizers.Fail(out x));
}
static bool LcApply(Input inp, ref Position pos, ref Rules rules, out LcTerm x)
{
var i = pos;
if (!rules.IsLoop(1) &&
Parse(inp, ref i, ref rules, out var lambda) &&
inp.WhiteSpaces(ref i) &&
Parse(inp, ref i, ref rules, out var arg))
{
x = new LcApply {
Lambda = lambda, Arg = arg
};
return(pos.Seek(i, ref rules));
}
return(Recognizers.Fail(out x));
}
static bool LcLambda(Input inp, ref Position pos, ref Rules rules, out LcTerm x)
{
var i = pos;
if (inp.Letters(ref i, out var v) &&
inp.Optional(inp.WhiteSpaces(ref i)) &&
inp.Literal("->", ref i) &&
Parse(inp, ref i, ref rules, out var body))
{
x = new LcLambda {
Var = v.ToString(), Body = body
};
return(pos.Seek(i, ref rules));
}
return(Recognizers.Fail(out x));
}