I'm new to SML and are playing with datatypes and such, I have this datatype for example:
datatype test = UInt of int
As far as I understand, UInt should work just like a normal int, but I cannot make it do simple things like sum 2 UInt:
val x = UInt(2)
val y = UInt(3)
val z = x + z
This code gives me this error:
= stdIn:4.11 Error: overloaded variable not defined at type
symbol: +
type: test
How can I make this work?
Thanks!
By default, + is defined for two types -- ints and reals, with type int * int -> int and real * real -> real, respectively. It is not defined for your type test.
You can redefine + to work on the test type as follows:
val (op+) = fn (UInt x, UInt y) => UInt (x + y)
However, redefining built-ins is generally not the best idea. You could also define a function that will take any int * int -> int function and give you a test * test -> test function that behaves as expected as follows:
fun lift f = fn (UInt x, UInt y) => UInt (f(x,y))
Using this method, you could define addition for your test type as val liftedAdd = lift (op+), and use this sort of function for all of your work with the test type.
However, there isn't much good reason to create a datatype that is simply a wrapper for an already existing type with no additional information. You might be looking for the type keyword, which will define a new type to be equal to one that already exists.
For example, if you say:
type test = int
val x : test = 2
val y : test = 3
val z = x + y
The addition will work, because while both x and y are tests, and + is defined on ints, test is simply an alias for int.