Question

Is there an inbuilt is integer function in sml?

I mean something like:

I have to read a number from file and display it as output if it integer and raise exception if the number is not integer. for example I have to check if the output of (Int.fromString()) is integer and display it if it is …(repeat above)

Answer 1

The type system will ensure that the types of the values a function is given match the type signature of the function.

That is, if you have a function that takes an integer as input, such as

fun double n = 2 * n
(* this function has type: int -> int *)

Then n will always be an integer. It will not be possible to call the function with anything but an integer; it will give a type error.

If you have a polymorphic function, such as

fun pair n = (n, n)
(* this function has type: 'a -> 'a * 'a *)

Then you cannot know what type the input is at runtime. All types of input will be treated the same.

You can, however, always restrict a polymorphic function to only work on a given type, by making the type explicit when defining the function:

fun pairInt (n : int) = (n, n)
(* this function has type: int -> int * int *)

You can see the difference between pair and pairInt by comparing what you get from calling pair 5 to pairInt 5, and pair "foo" to pairInt "foo".

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If you have an int option, as is the case if you try to convert an string to a int using Int.fromString, you can extract the int in several ways. For how to do this, I refer you to the question “In smlnj how do you convert “string option” to “string”?“.