Limited support for nominal typing in Motoko

Developers Language Support
1

Are there any plans to add limited support for nominal (as opposed to structural) typing in Motoko?

So that would let us define two separate types:

type Kilometer = Float;
type Mile = Float;

without the Motoko type checker treating them as equivalent.

2

You could do something like this:

type Kilometer = { #kilometer : Float };
type Mile = { #mile : Float };

This is an approximation of what some languages call a “newtype” or “newtype wrapper”.

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3

Hmm yeah that’s actually not a bad idea.

4

FWIW there used to be some runtime constructs to this effect (as evidenced by the following issue)

5

Oh wow, didn’t know Motoko dated back to 2018.

Going back to the newtype idiom, I wish Motoko had the “tuple struct” construct that Rust has, so I could omit the #kilometer in type Kilometer = { #kilometer : Float };…

6

Without the tag you wouldn’t be able to construct a value of that type though.

7

Well, not being able to omit it is the whole point of the pattern, since it determines and makes explicit which unit is being used.

FWIW, a singleton record {km : Float} would work just as well.

There never were nominal types in the language, I believe we were merely hypothesising on that issue. Nominal types are rather problematic in a distributed setting, since somebody will “own” their definition locally. That is, you cannot really use them easily for messaging. So it was natural to avoid them altogether.

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