Annotate immediately-invoked functions for inlining flow control analysis
See original GitHub issuePer #9998 and its many offshoots, we have problems analyzing code like this
let x: string | number = "OK";
mystery(() => {
x = 10;
});
if (x === 10) { // Error, we believe this to be impossible
}
The problem is that we don’t know if mystery
invokes its argument “now” or “later” (or possibly even never!).
The converse problem appears during reads:
let x: string | number = ...;
if (typeof x === 'string') {
// Error, toLowerCase doesn't exist on string | number
let arr = [1, 2, 3].map(n => x.toLowerCase());
}
Proposal is to allow some form of indicating that the function is invoked immediately, e.g.
declare function mystery(arg: immediate () => void): void;
This would “inline” the function body for the purposes of flow control analysis
Issue Analytics
- State:
- Created 7 years ago
- Reactions:112
- Comments:59 (30 by maintainers)
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@oswaldofreitas
You can write:
I’m sure you have considered this at length but given the original example
I think the most intuitive behavior would be for a closure which rebinds
x
to cause the type ofx
to revert tostring | number
.As @kitsonk remarked
While I am in favor of what CFA brings especially in catching logic errors and redundant (superstitious) checks, I think the example above should not be an error because the caller is justified in checking x against
10
simply becausemystery
may have invoked the closure but it may not have invoked the closure. Consider a case where mystery invokes the closure immediately, but conditionally based on some other arbitrary state. consider:EDIT fixed typo, formatting