undefined | Better HN

0 pointsrolfvandekrol3y ago0 comments

compose is a higher order function. In the first step it accepts a function that converts 3 values (V1 to V3) into a single values (T1) and a series of conversion functions that converts this single value into another value (T1 into T2, T2 into T3 and so on until T6). Using these functions it produces a new function that converts the combination of V1, V2 and V3 into a T6.

I don't know ramda, but I assume this is only part of the type definition of compose and this is just the longest part of it. I think compose is written in such a way that it can accept a many conversion functions as you want and this is just the longest variant that is encoded in the types.

0 comments

solardev3y ago

Yes, that's correct... compose() pipes a bunch of passed functions from right to left. Here's the actual type definition if you want to see: https://github.com/googol/DefinitelyTyped/blob/6836f798cb186...

But even at its simplest variant, with just one or two functions passed, what the V0 or T1 do is pretty confusing. I thiiiiiiiiink it's trying to ensure the return type of one function is correctly passed as the input type of the subsequent function, and so on and so forth, but I don't really know.

Also, I should note that I'm using an older version of that lib. The latest version has cleaner typings... still difficult, but at least formatted better: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/mast...

rolfvandekrolOP3y ago

It indeed ensures one a type level that this series of functions can work as a series so every subsequent function can accept the return type of the previous one.

The latest version seems to have abandoned the typing of the initial input, which makes the types a little simpler.

My point however is that the types you point our here are actually not particularly complex. They are just long, with lot's of inputs for the generics (and the syntax may be confusing). The types in the original article are much more complicated, with conditional type inference etc.

_the_inflator3y ago

Great and simple explanation. Suddenly it doesn't feel cryptic, simply logical. Thx!

j / k navigate · click thread line to collapse