SML shorter base case syntax for my function

Question

What would be a shorter syntax for this function? I'm noticing that a lot of programmers use pipes in their functions, but I'm stuck trying to figure out how I could do the same for this one.

fun test (i, x) = 
  if null x
then false
else
  if hd x = i then true
  else test(i, tl x)
//
val test = fn : ''a * ''a list -> bool

Answer 1

Here is a shorter version:

fun test (_, []) = false
  | test (i, x::xs) = (x = i) orelse test (i, xs)

Here are the things I did:

1. Use pattern matching. In your second solution you're already pattern matching against the empty list, so this takes it a step further and pattern matches on the non-empty list (x::xs), making the use of hd and tl unnecessary.

2. Use the _ pattern when you don't need the value.

3. Instead of if P then true else Q, write P orelse Q.

The parenthesis around x = i isn't strictly necessary. It looks a little confusing with the two different =s next to one another (one comes from the function definition, and the other is a binary operator), but this is also perfectly valid and means the same:

fun test (_, []) = false
  | test (i, x::xs) = x = i orelse test (i, xs)

Or indented a bit more conveniently:

fun test (_, []) = false
  | test (i, x::xs) =
      x = i orelse test (i, xs)