< system > Iterables in base or inherit from context for dynamic functions

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1

I just finished refactoring a large function to Array.tabulate to try to save some cycles and heap(see New Vector data structure in Motoko - #5 by skilesare) and received a rude awakening when I realized I had to call a type function to create a timer.

Generally, it seems we need a way to do <system> in iterable functions like tabulate, map, fold, etc.

Would it be insecure to have dynamic functions inherit the type of their context function? I added system to the function that calls tabulate but it doesn’t flow through:

public func myfunc<system>(caller: Principal, items : [Items]) : async* [?result]{

      //doesn't work 
      let results = Array.tabulate(items.size(), func<system>(i : Nat): ?Items {

      //doesn't exist and calls into Prim so I can't shim my own
      let results = Array.tabulateSys<system>(items.size(), func<system>(i : Nat): ?Items {

      //preferred - just flows through to dynamic functions because containing function has the `system` type.
       let results = Array.tabulate(items.size(), func(i : Nat): ?Items {

Pingning @claudio @luc-blaeser @ggreif @rvanasa @dfx-json

2

Would it be insecure to have dynamic functions inherit the type of their context function? I added system to the function that calls tabulate but it doesn’t flow through:

I think it would because then you could return a function that parades as a safe function but needs system capability.

Here’s a way to define a system version of tabulate, though slightly clunky:

import Array "mo:base/Array";
import Debug "mo:base/Debug";


actor {

   func tabulate<system, T>(n : Nat, f : <system>Nat -> T) : [T] {
      let a = Array.init<?T>(n, null);
      for (i in a.keys()) {
        a[i] := ?f<system>(i);
      };
      Array.tabulate<T>(n, func(i) { 
        let ?v = a[i] else Debug.trap("impossible");
        v
      });
   };

   func protected<system>(n : Nat) : Nat {n};

   let a = tabulate<system, Nat>(100, func<system>(n : Nat) : Nat { protected<system>(n);} );

}
3

Many thanks for the example! I’ll try to run some performance on it…I’m expecting it to lose a good bit of the performance advantages it has in the performance metrics of the ‘standard’ tabulate as mentioned on Mops • Motoko Package Manager, but we’ll have to see by how much!

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