public async Task <Value> Visit(Apply apply)
{
// Find out what the lambda is, then evaluate it with the given expression
// Do we do a syntactic substitution?
// evaluate our lambda until we get an LambdaValue. Then we substitute and continue.
var lambda = await apply.Lambda.Accept(this);
if (lambda is LambdaValue)
{
LambdaValue val = (LambdaValue)lambda;
VariableSubstituter subst = new VariableSubstituter(val.VariableName, apply.Expression);
var newExpr = subst.Substitute(val.Expression);
return(await newExpr.Accept(this));
}
else
{
throw new ArgumentException("Value cannot be evaluated to a Lambda");
}
//VariableSubstituter subst = new VariableSubstituter(apply.va)
throw new NotImplementedException();
}
public async Task <Value> Visit <A>(IRecExpression <A> rec) where A : Expression
{
// A rec has an input, a start, and a step, which itself has two free vars
// The idea is to find out what value the input is.
// if it is zero we return the start
// otherwise we evaluate the rec with an input one less, then put this through the step
Value input = await Run(rec.Input); // we need this
if (input is NumValue)
{
NumValue num = input as NumValue;
if (num.Value == 0)
{
return(await Run(rec.Start));
}
else
{
// this looks bad. Fix it
int predecessor = num.Value - 1;
NumValue newNum = new NumValue(predecessor);
VariableSubstituter substNum = new VariableSubstituter(rec.NumVariableName, new NumConstant(predecessor));
Expression newRec = rec.Construct(new NumConstant(predecessor), rec.Start, rec.NumVariableName,
rec.AccVariableName, rec.Step);
VariableSubstituter substAcc = new VariableSubstituter(rec.AccVariableName, newRec);
Expression temp = substNum.Substitute(rec.Step);
Expression temp2 = substAcc.Substitute(temp);
return(await Run(temp2));
}
}
else
{
throw new ArgumentException("In rec the input should evaluate to a Num");
}
throw new NotImplementedException();
}