Why ReadonlyXyz?

[edalex-ian]

In responding to #1608 @mlegenhausen highlighted to me the abundance and use of the ReadonlyXyz type classes (ReadonlyNonEmptyArray, ReadonlyRecord, ReadonlyMap, etc.). But I was under the impression that the non-ReadonlyXyz type classes were following the immutable paradigm. Is that not correct?

Or, if that is correct then what's the intention behind these? Documentation purposes?

[mlegenhausen]

Me again 😅

But I was under the impression that the non-ReadonlyXyz type classes were following the immutable paradigm

This only holds if you use fp-ts functions for data manipulations, but not for the functions that are defined on the non-readonly interfaces. An Array still has a push function which will mutate the array. On an ReadonlyArray the push function is not defined at all. So using the Readonly variants gives you simply more type safety, but with a "cost" that you need to explicitly convert to a mutable array when e. g. calling APIs that expects a mutable data structure.

In fp-ts@3 the mutable versions of Array, Record, NonEmptyArray and Map will be deprecated and will only contain functions to convert from and to readonly versions of these data structures. See Issue #1226.

[edalex-ian]

Thanks again @mlegenhausen .

Just to make sure I understand you, where you say:

An Array still has a push function function which will mutate the array.

Here you're referring to standard JS Array, and so really fp-ts Array is not changing the underlying type - just providing a thin veneer over it to allow FP idioms to be used. Whereas with ReadonlyArray we now have a different type, and so fp-ts has full control?

[mlegenhausen]

Here you're referring to standard JS Array, and so really fp-ts Array is not changing the underlying type - just providing a thin veneer over it to allow FP idioms to be used.

Yes, and if you take a look at the Array source you will not find any Array definition cause the standard typescript Array datatype is used.

Whereas with ReadonlyArray we now have a different type, and so fp-ts has full control?

No, it is just the same as with the Array. ReadonlyArray is a datatype shipped with TypeScript, but it simply fits better with functional programming due to the guarantees you get from immutability.

[edalex-ian]

Ah! I'd missed that typescript had a ReadonlyArray - and I now see since 3.4 you can also just use the readonly modifier.

Thanks for the clarification @mlegenhausen 👍