sml

Converting int to real in SML

Newbie at SML

I have the following code that returns the absolute value of a list. I need it to be of type int list -> real list. Where do I place the statement that converts it to real while constraining this code to a single line?

val myabs = map(fn x => if x >= 0 then x else ~x) [1,~2, 3, ~4];
Solution

  • You convert an int to real using Real.fromInt:

    - Real.fromInt 42;
> val it = 42.0 : real

    You can convert an int list into a real list by List.map Real.fromInt:

    - List.map Real.fromInt [1, 2, 3];
> val it = [1.0, 2.0, 3.0] : real list

    You can convert an integer to its absolute using Int.abs:

    - Int.abs ~42;
> val it = 42 : int

    You can combine those two functions and so both convert an integer to its absolute and convert it to real:

    - (Real.fromInt o Int.abs) ~42;
> val it = 42.0 : real

    And you can do this for an entire list using List.map (Real.fromInt o Int.abs):

    - List.map (Real.fromInt o Int.abs) [~1, ~2, ~3];
> val it = [1.0, 2.0, 3.0] : real list

    You can express that as a single function:

    fun myabs xs = List.map (fn x => Real.fromInt (Int.abs x)) xs

    And you can shorten this function a bit:

    val myabs = List.map (fn x => Real.fromInt (Int.abs x))
val myabs = List.map (fn x => (Real.fromInt o Int.abs) x)
val myabs = List.map (Real.fromInt o Int.abs)

    So the only missing pieces were: