SML How to check variable type?

Question

Is there any way to check/test the type of a variable?

I want to use it like this:

if x = int then foo
else if x = real then bar
else if x = string then          


        

Answer 1

ML languages are statically typed, so it's not possible for something to have different types at different times. x can't sometimes have type int and at other times have the type string. If you need behavior like this, the normal way to go about it is to wrap the value in a container that encodes type information, like:

datatype wrapper = Int of int | Real of real | String of string

Then you can pattern-match on the constructor:

case x of Int x    -> foo
        | Real x   -> bar
        | String x -> ...

In this case, x is clearly typed as a wrapper, so that will work.