Explain why Readonly array is not good idea

[steida]

📖 Documentation

Readonly arrays are great to prevent accidental mutations via sort method, but PITA when used with code requiring normal Array. Long story short, it's: Use it everywhere, nowhere, or cast all the time.

I didn't find any other frictionless solution from the DX perspective than not using readonly arrays at all.

I believe it should be mentioned in the documentation.

[raveclassic]

It's ok to accept ReadonlyArray or even ArrayLike as an argument but things get worse when using them as a return type.

I think the best solution so far is to accept ArrayLike and return Array if a function constructs a new Array.

Note: we have the same problem in most mostjs/core#296 (comment)

[steida]

@gcanti ReadonlyArray really should be banned in fp-ts ecosystem. It's only pain without any measurable benefit. I just spend another hour with export interface Context extends ReadonlyArray<ContextEntry> { trying to figure out why I can not use fp-ts Array. Not I have to cast (ugly) or call slice(0) (ugly too).

[gcanti]

ReadonlyArray really should be banned in fp-ts ecosystem

In practice it already is (except for io-ts's Context, for legacy reasons)

[andrei-zhidkov]

Sorry, but I don't get why the use of ReadonlyArray (a built-in type) should be discouraged in the fp-ts docs.
Besides, I'm not sure I understand the reason to artificially limit the ReadonlyArray usage, given that the functions in Array.ts are pure.

ReadonlyArrays are great, and I'd be glad to submit a PR to support them more (either with overloads to return ReadonlyArrays when operating on ReadonlyArrays or using ArrayLike and returning Arrays as @raveclassic suggested) if that's ok with the maintainers.

[steida]

"ReadonlyArrays are great" if everything is read-only array otherwise poor developers have to cast them all the time. If fp-ts people are happy with it casting all the time or not using mutable arrays at all, then ok.

[SRachamim]

In our projects we're enforcing using readonly and ReadonlyArray with functional linting rules.

[gcanti]

Actually I'm considering to drop support for mutable modules in v3, #1226