How can I assign user defined sml datatypes to have a integer (as string) datatype?

My problem is that I'm trying to make my own datatype, but I can not use ints in the naming convention of the items that are appart of the datatype

datatype psu = 350w | 450w | 550w | ... etc;

error thrown:

stdIn:16.15-16.30 Error: syntax error: deleting  STRING BAR STRING

Is there some way I can convert these numbers into a string that would be acceptable for the sml interpreter? I really don't want to name my variable "three hundred and fifty watts" or anything.

I tried looking up a toString and just making the variables explicitly strings, but I could not find a helpful toString and making the variables strings just threw another error

Solution

As noted in comments, identifiers in SML (as in many other programming languages) cannot begin with digits.

datatype psu = W350 | W450 | W550 | ...

This would work.

Something like the following would also work, but does not let you constrain the possibilities.

datatype psu = Watts of int

Similarly, if you represent that value as a string like "350W" you cannot ensure at compile-time that the value held in that string is in a particular set of options.