Ts-toolbelt: Why didn't you create a 'Defer' type alias?
馃 Question
In other discussions, you mention the use of extends infer X syntax as a way to defer evaluation of a type by the typescript compiler until we've received type parameters.
Discussion here: https://github.com/pirix-gh/ts-toolbelt/issues/102
And here: https://github.com/pirix-gh/ts-toolbelt/issues/5
Describe your question
Is there a reason you've directly used this syntax in all of your types and haven't created a 'Defer' type, such as type Defer<T, TAs = T> = T extends infer X ? Cast<X, TAs> : never;?
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typescript #deferred #type-evaluation
martaver
All 2 comments
Because if you would do that... Let's suppose you have a recursive type A. Then you want a deferred type A. If you did type DeferA<...> = Defer<A<...>>, then the type A<...> would be expanded by TypeScript before it's even passed to Defer<...>, thus making Defer pointless.
TypeScript tries to expand types as it traverses them. ... extends infer X is the only way to prevent it from doing this. In essence, a Defer type won't work because it will expand (compute) the recursive type before it has even been passed to Defer.
millsp
on 4 Apr 2020
Of course... thanks for the break down!
martaver
on 4 Apr 2020
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Most helpful comment
Because if you would do that... Let's suppose you have a recursive type
A. Then you want a deferred typeA. If you didtype DeferA<...> = Defer<A<...>>, then the typeA<...>would be expanded by TypeScript before it's even passed toDefer<...>, thus makingDeferpointless.TypeScript tries to expand types as it traverses them.
... extends infer Xis the only way to prevent it from doing this. In essence, aDefertype won't work because it will expand (compute) the recursive type before it has even been passed toDefer.