Many languages provide for a ForStatement in one or other form. Although most people are familiar with these, their semantics and implementation can actually be quite tricky.
Suppose we were to add a simple Pascal-style for loop to Parva, to allow statements with concrete syntax defined by
ForStatement = "for" Designator "=" Expression ("to" | "downto") Expression "do" Statement .
for example
for i = 1 to 10 do write(i); // forwards 1 2 3 4 5 6 7 8 9 10
for i = 10 downto 1 do write(i); // backwards 10 9 8 7 6 5 4 3 2 1
for i = 10 to 5 do write(i); // no effect
for b = false to true do write(b); // writes "false true"
for i = i - 1 to i + 6 do write(i); // what does this do?
for i = 1 to 5 do read(i); // should we allow this?
for i = 1 to 5 do i = i + 1; // should we allow this?
What would you suggest in the way of constraints, how would you enforce the constraints, and how would you generate code for these sorts of statements?