private static Expression ParseSExpression(SExpression sExpression)
{
var expressions = sExpression.Tokens.Select(ct => ParseExpression(ct)).ToArray();
if (expressions.Any() && expressions.First() is SpecialFormToken)
{
return(ParseSpecialForm((expressions.First() as SpecialFormToken).Token, expressions.Skip(1).ToList(), sExpression));
}
return(new Application(expressions));
}
private static Expression ParseSpecialForm(string specialForm, List <Expression> expressions, SExpression sExpression)
{
if (specialForm == "if")
{
if (expressions.Count < 2)
{
throw new ParseException(sExpression.ToString(), "An \"if\" expression must contains at least 2 arguments");
}
if (expressions.Count > 3)
{
throw new ParseException(sExpression.ToString(), "An \"if\" expression must contains at most 3 arguments");
}
var falseBranch = expressions.Count == 3 ? expressions[2] : new PrimitiveWrapper <bool>()
{
Value = false
};
return(new If(expressions[0], expressions[1], falseBranch));
}
if (specialForm == "define")
{
if (expressions[0] is Variable)
{
if (expressions.Count != 2)
{
throw new ParseException(sExpression.ToString(), "A variable type \"define\" expression must contains exactly 2 arguments");
}
return(new Define(expressions[0], expressions[1]));
}
if (expressions[0] is Application)
{
if (expressions.Count == 1)
{
throw new ParseException(sExpression.ToString(), "A procedure type \"define\" expression must contains at least 2 arguments");
}
var name = expressions[0];
var defined = MakeBeginIfNeeded(expressions.Skip(1).ToList());
return(new Define(name, defined));
}
throw new ParseException(sExpression.ToString(), "A \"define\" expression's first part must be either a string or a list");
}
if (specialForm == "lambda")
{
if (expressions.Count == 1)
{
throw new ParseException(sExpression.ToString(), "A \"lambda\" expression must contains at least 2 arguments");
}
var defined = MakeBeginIfNeeded(expressions.Skip(1).ToList());
if (!(expressions[0] is Application))
{
throw new ParseException(sExpression.ToString(), "A \"lambda\" expression's first part must be a list");
}
var formalArgs = (expressions[0] as Application).Expressions;
if (formalArgs.Any(e => !(e is Variable)))
{
throw new ParseException(sExpression.ToString(), "A \"lambda\" expression's first part must be a list of arguments");
}
return(new Lambda((expressions[0] as Application).Expressions.Cast <Variable>().ToArray(), defined));
}
if (specialForm == "let" || specialForm == "let*")
{
if (expressions.Count < 2)
{
throw new ParseException(sExpression.ToString(), string.Format("A \"{0}\" expression must contains at least 2 arguments", specialForm));
}
if (!(expressions[0] is Application))
{
throw new ParseException(sExpression.ToString(), string.Format("A \"{0}\" expression's first part must be a list", specialForm));
}
var tuples = new List <Tuple <Variable, Expression> >();
foreach (var expression in (expressions[0] as Application).Expressions)
{
if (!(expression is Application && (expression as Application).Expressions.Length == 2))
{
throw new ParseException(sExpression.ToString(), string.Format("A \"{0}\" definition must be a two-element list", specialForm));
}
var applcation = expression as Application;
if (!(applcation.Expressions[0] is Variable))
{
throw new ParseException(sExpression.ToString(), string.Format("A \"{0}\" definition must begin with a variable", specialForm));
}
tuples.Add(new Tuple <Variable, Expression>(applcation.Expressions[0] as Variable, applcation.Expressions[1]));
}
var body = MakeBeginIfNeeded(expressions.Skip(1).ToList());
if (specialForm == "let")
{
return(new Let(tuples, body));
}
else if (specialForm == "let*")
{
return(new LetStar(tuples, body));
}
}
if (specialForm == "begin")
{
return(new Begin(expressions));
}
if (specialForm == "cond")
{
var last = expressions.Last() as Application;
Expression condition = last.Expressions[0];
if (last.Expressions[0] is SpecialToken && (last.Expressions[0] as SpecialToken).Token == "else")
{
condition = new PrimitiveWrapper <bool>()
{
Value = true
}
}
;
var accIf = new If(condition, last.Expressions[1], Void.Instance);
return(expressions.Cast <Application>().Reverse().Skip(1)
.Aggregate(accIf, (acc, x) => new If(x.Expressions[0], x.Expressions[1], acc)));
}
if (specialForm == "delay")
{
var expr = expressions.Single();
return(new Promise(expr));
}
throw new InternalException(string.Format("Illegal special form {0}", specialForm));
}
