Do ? inside lambda functions

Developers
1

When we put a lambda function inside a do ? block, e.g. Array.filter<T>(arr, func(ele) { ... }), the compiler doesn’t let use a ! inside the lambda because there’s no enclosing do ?.

That makes it hard to use some library functions inside a do ? block.

Is there an easy way around this I’m missing?

2

That’s right, you cannot access a do? block from inside an inner function (in the same way you cannot jump to an outer label or return from an outer function).

If this was allowed, the ! could escape the do? block, because you could write examples like the following:

var f : ?Nat -> Nat = func(x : ?Nat) { 0 };  // dummy initialiser
let o = do? {
  f := func(x : ?Nat) { x! };
};
f(null);  // what now?

and various other variations.

So we cannot allow it. There is no easy workaround, I’m afraid – do? blocks are convenient, but do not always apply.

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3

Ah I see, so the issue lies in the difference in where a function is defined and where it is called. Thanks for clarifying.

4

Just to follow up on this, it seems like the Motoko compiler is totally fine with trapping (i.e. assert false) inside a lambda (i.e. inner function), and that will trap the executing canister method as expected. Can you confirm if that’s OK to do?

5

Yes, a trap can potentially occur at any point during execution (in Motoko or otherwise), and it aborts the current canister method.

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