AssemblyScript
Map.get() always returns error, no way to catch it?
Question
I'm writing an AssemblyScript version of Conways Game of Life - https://github.com/KieranP/Game-Of-Life-Implementations
Part of it needs to fetch values from a Map but the key may not be there.
private cell_at(x: u32, y: u32): Cell {
return this.cells.get(`${x}-${y}`)
}
However, AssemblyScript always throws an error instead of returning null.
https://github.com/AssemblyScript/assemblyscript/blob/8257b1c60b6dc9a6a1395236b95a82efc261e3b8/std/assembly/map.ts#L103-L107
Map.find() is a private method, so I can't use that. And trying to use try/catch throws error ERROR AS100: Not implemented: Exceptions.
So how am I supposed to do this, try and fetch a map value, and handle when the map doesn't have the expected value?
According to the docs (https://www.assemblyscript.org/stdlib/map.html), it recommends using has first:
private cell_at(x: u32, y: u32): Cell | null {
const key = `${x}-${y}`
if (this.cells.has(key)) {
return this.cells.get(key)
} else {
return null
}
}
But that obviously adds 2x the function invocations, both using find.
It would be nice if Map.get allowed a second argument to specify the fallback value.
get(key: K, fallback: F): V {
let entry = this.find(key, HASH<K>(key));
if (!entry) {
if (fallback) return fallback
throw new Error(E_KEYNOTFOUND); // cannot represent `undefined`
}
return entry.value;
}
private cell_at(x: u32, y: u32): Cell | null {
return this.cells.get(`${x}-${y}`, null)
}
I agree with you. In the original design, I think AS just wants to avoid undefined because according to TS, here should be T | undefined instead of T | null.
@MaxGraey @dcodeIO @CountBleck Do you think we can return T | null for map.get? Though it is different with TS but it has better performance.
Issues in play here:
- A
Map<K,i32>would need to returni32 | null, which is an invalid type. - A
Map<K,V | null>would have an ambiguousnullreturn. - Not all (future) Wasm types might be nullable.
Providing a second argument to specify a default could work, but I think we'd need a separate method for it, say getWithDefault to not break the existing behavior.
I'd prefer a TryGet or MaybeGet pattern that gives both a boolean indicator of success and a value in the case of success.
A GetWithDefault pattern has the problem of not being able to distinguish between the default value existing at the key in the map, or the entry not existing at all.
