Thread by @devanshj__

🎁 New Year's Gift for Y'all

❓ `@​sthir/predicate` — An eDSL to write typed predicates

🧙🏻‍♂️ Write your logical expression in a JS-like eDSL and get the typed predicate inferred.

🔗

👇🏻 Let me explain...github.com/devanshj/sthir…

We often write predicates, for example while filtering an array. But its type (as in `x is NarrowedX`) doesn't get inferred...

Moreover TypeScript is a little conservative, if you were writing this in JavaScript you won't write `"a" in x`, you'd simply write `typeof x.a === "string"`, wouldn't you?

With `@​sthir/predicate` you can simply import `p` and write the predicate...

💆🏻‍♂️ in JS-like DSL
💪🏻 with autocompletions
🥵 and the typed predicate gets inferred automatically!
👉🏻 Try it yourself tsplay.dev/wEGZgm

And you know what's the best part? With you can use the macro version `@​sthir/predicate/macro` with `babel-plugin-macro` (thanks to @kentcdodds) for zero runtime overhead.

If you use the macro version in the above code, it'll get transformed to this in build-time! 🤠

You can also use the predicate in any other places that affect the control-flow analysis, for example in an `if` statement...

And that too would be inlined with the macro

tsplay.dev/mZbkJm

And I see ya'll get bummed here on twitter when TypeScript doesn't infer the predicate and doesn't narrow the types. Let's look at a few examples and see how `@​sthir/predicate` fixes it 👇🏻

🤔 Problem: x.com/wcandillon/sta…

😏 Solved: tsplay.dev/NddqyN

Now there is such a library, @wcandillon 😎

🤔 Problem: x.com/addaleax/statu…

😏 Solved: tsplay.dev/NVgGvm

@addaleax I hope this makes you love TS even more! 😁

🤔 Problem: x.com/AdamRackis/sta…

😏 Solved: tsplay.dev/w2EJYW

There you go @AdamRackis (not stupid for asking that question but still stupid for thinking it was a great decision to make `Promise` non-monadic 😛)

That's it folks, enjoy!

If you think `@​sthir/predicate` is dope, don't forget to RT to share it with your friends 🤪

cc @typescript @drosenwasser @SeaRyanC Next time someone complains about not inferring typed predicates send them here 👆🏻😏😎

Also I forgot to pitch how `@​sthir/predicate` makes your code more type-safe than the code that doesn't use it 👇🏻👮🏻‍♂️

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