How to declare inputs of a curried function as real in sml?

Question

Here is my code for a curried factorial function I want the output as real

fun pow(x:real) (n:real)= if (n=0.0) then 1.0 else x:real*pow(x:real) (n-1:real) ;

But my syntax is really really wrong how do I fix this?

The following Q&A: [Why can't I compare reals in Standard ML?](https://stackoverflow.com/questions/41394992/why-cant-i-compare-reals-in-standard-ml) has the `real` factorial function as its example (although rewritten into pattern matching). But I admit ruakh's `real` solution below which avoids the comparison is pretty neat, too. — sshine, Feb 24 '20 at 09:59
Why not use the built-in `Math.pow : real * real -> real`, though? — sshine, Feb 24 '20 at 15:13

score 1 · Accepted Answer · answered Feb 23 '20 at 06:33

I think what you want is:

fun pow x n =
  if n = 0
  then 1.0
  else x * pow x (n - 1)

or, if you want to be a bit more explicit about the types:

fun pow (x : real) (n : int) : real =
  if n = 0
  then 1.0
  else x * pow x (n - 1)

That is:

I think you want n to be of type int, not type real. (Your approach only makes sense if n is a nonnegative integer, since otherwise the recursion would just go forever.)
You don't need so many :real-s everywhere; they don't add anything, because the compiler can infer the types.

1 Answers1