variable gets forgotten after print statement

Question

Why does this compile?

fun foo (h::t) =
    h = hd(t);

But this does not

fun foo (h::t) =
    PolyML.print (h::t);
    print "\n";
    h = hd(t);

?

Value or constructor (h) has not been declared   Found near =( h, hd(t))

Value or constructor (t) has not been declared   Found near =( h, hd(t))
Exception- Fail "Static errors (pass2)" raised

Answer 1

I think your frustration with the language prevents you from solving your problem(s) more than the limitations of the language. As I said in a previous answer, semicolons cannot be used like you used them. You need to wrap those statements inside parentheses:

fun foo (h::t) =
  (
    PolyML.print (h::t);
    print "\n";
    h = hd(t)
  )

Furthermore, you first snippet doesn't need a semicolon:

fun foo (h::t) =
    h = hd(t)

Here's the thing, in SML semicolons are not used to terminate statements, they're used to separate expressions. Think of ; as a binary operator, just like + or -. With the added constraint that you need parentheses around.

Also, you're probably using the = operator in the wrong way inside h = hd(t). It's not assignment, it's an equality check, just like == in other languages. If you want assignment, you need a ref type.

It's probably better to ask what exactly you're trying to solve, because at this point you're totally misunderstanding the syntax and semantics of SML and we can't really write a tutorial on in here.