Docs typo in Language Quick Reference / Subtyping?

Developers Language Support
1

Some of the conditions for a shared function of type T: T1 -> T2 to be a subtype of another shared function U: U1 -> U2 are said to be:
U1 <: T1
T2 <: U2

This means that:

  • The arguments of T must be a supertype of the arguments of U
  • The return type of T must be a subtype of the return type of U

But when testing in Motoko Playground and DFX only the reverse condition allows upgrading without a backwards compatibility warning:
T1 <: U1
U2 <: T2

This last condition, where arguments can ‘grow’ in type and return types can only ‘shrink’ in type, is the one that makes most sense to clients and implementers of functions.

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2

Oops my bad!

Spent a whole day just to discover that I had things switched up :joy:

I tried to upgrade T to U, but I had to to the opposite.

The docs are correct!

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