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Simpler Expression of Complex Type Conditions #41123
Description
Search Terms
Multiple, type, condition, expression, no, extra, case, and, or
Suggestion
I often find myself repeating sub-branches of conditional types, which makes reasoning about control flow more difficult. I'd recommend the same logical operators to those of JS land, at the type-level.
Example
Let's say we have three boolean types––ConditionA, ConditionB and ConditionC––and we want to gather the number of true conditions in a new type Result, which will evaluate to 0 | 1 | 2 | 3 depending on the aforementioned conditions. The runtime equivalent can be expressed quite legibly:
const Result =
(conditionA && conditionB && conditionC)
? 3
: (
(conditionA && conditionB) ||
(conditionB && conditionC) ||
(conditionA && conditionC)
)
? 2
: (conditionA || conditionB || conditionC)
? 1
: 0;The type-level equivalent is not as obvious in its meaning, and we end up repeating the potential results.
type Result =
ConditionA extends true
? ConditionB extends true
? ConditionC extends true
? 3
: 2
: ConditionC extends true
? 2
: 1
: ConditionB extends true
? ConditionC extends true
? 2
: 1
: ConditionC extends true
? 1
: 0;While it nets more code, the following is––imo––easier to understand.
type Result =
(ConditionA extends true && ConditionB extends true && ConditionC extends true)
? 3
: (
(ConditionA extends true && ConditionB extends true) ||
(ConditionB extends true && ConditionC extends true) ||
(ConditionA extends true && ConditionC extends true)
)
? 2
: (ConditionA extends true || ConditionB extends true || ConditionC extends true)
? 1
: 0;Ideally, the same rules from JS-land could apply to judging whether a type is a subtype of truthy, so we could do this:
type Result =
(ConditionA && ConditionB && ConditionC)
? 3
: (
(ConditionA && ConditionB) ||
(ConditionB && ConditionC) ||
(ConditionA && ConditionC)
)
? 2
: (ConditionA || ConditionB || ConditionC)
? 1
: 0;Checklist
My suggestion meets these guidelines:
- This wouldn't be a breaking change in existing TypeScript/JavaScript code
- This wouldn't change the runtime behavior of existing JavaScript code
- This could be implemented without emitting different JS based on the types of the expressions
- This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
- This feature would agree with the rest of TypeScript's Design Goals.
Activity
tjjfvi commentedon Nov 4, 2020
+1
Additionally, I think it should allow the below, for parity with intersection / union and also cleaner multiline conditionals
I really like this for when I have long chains of
A ? B ? C ? T : never : never : never;(A && B && C) ? T : neverseems a lot cleaner.I don't particularly care for requiring parens, but if it's necessary for avoiding ambiguity, it's still better than what we have right now.
tjjfvi commentedon Nov 11, 2020
Also, for conditions consisting solely of
&&s,infer Ts should be allowed, and be processed left to right, i.e.:Should be valid.
harrysolovay commentedon Nov 11, 2020
T6 (@tjjfvi) absolutely! Great idea.