了解sml语言的语法

Question

Hello I started to write in sml and I have some difficulty in understanding a particular function.

I have this function:

fun isInRow (r:int) ((x,y)) = x=r;

I would be happy to get explain to some points:

1. What the function accepts and what it returns.

2. What is the relationship between (r: int) ((x, y)) .

Thanks very much !!!

Answer 1

The function isInRow has two arguments. The first is named r . The second is a pair (x, y) . The type ascription (r: int) says that r must be an int.

This function is curried, which is a little unusual for SML. What this means roughly speaking is that it accepts arguments given separately rather than supplied as a pair.

So, the function accepts an int and a pair whose first element is an int. These are accepted as separate arguments. It returns a boolean value (the result of the comparison x = r ).

A call to the function would look like this:

isInRow 3 (3, 4)

There is more to say about currying (which is kind of cool), but I hope this is enough to get you going.

Answer 2

In addition to what Jeffrey has said,

• You don't need the extra set of parentheses:

   fun isInRow (r:int) (x,y) = x=r; 

• You don't need to specify the type :int . If you instead write:

   fun isInRow r (x,y) = x=r; 

  then the function's changes type from int → (int • 'a) → bool into ''a → (''a • 'b) → bool , meaning that r and x can have any type that can be compared for equality (not just int), and y can still be anything since it is still disregarded.

  Polymorphic functions are one of the strengths of typed, functional languages like SML.

• You could even refrain from giving y a name:

   fun isInRow r (x,_) = x=r;