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

Question

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

An identifier can't begin with a digit. What's wrong with datatype psu = W350 | W450 | W550 | ..., or datatype psu = Watts of int? What does this type represent and how are you going to use it? — molbdnilo
– molbdnilo, Commented Apr 1, 2022 at 6:13
BTW: that definition does not produce that error. Your actual definition said datatype psu = "350w" | "450w" | .... I think you've been so focused on strings that you're becoming stuck in them as a solution, but they rarely are. — molbdnilo
– molbdnilo, Commented Apr 1, 2022 at 6:21
Even I was about to suggest this..datatype psu = W350 | W450 | W550 | ... — Nalin Ranjan
– Nalin Ranjan, Commented Apr 1, 2022 at 6:34

Chris · Accepted Answer · 2022-04-01 06:35:16Z

1

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.

answered Apr 1, 2022 at 6:35

Chris

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1 Comment

nellyjelly56 Over a year ago

I did not realize that identifiers in sml could not start with digits. I guess I will use the W350 style for my program. Thank you!

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