Repr vs ToString

Stream: general

[Violeta Hernández]

Say there's a type with two sorts of textual representations, one of which is easy to parse back but hard to read, and the other which is harder to parse back but much easier to read.

[Violeta Hernández]

Am I correct in saying the former should be a Repr instance and the latter should be ToString?

[Violeta Hernández]

Or, how does this generally work?

[Sofia Rodrigues]

Well, I believe Repr is meant to represent the exact code used to build the structure. For example, in the datetime library PR, Markus (one of the members of Lean FRO leading the efforts to improve the Std) told me to make the Repr of a DateTime look like date("2024-10-24T13:05:20") that is exactly the macro used to create the DateTime and then to use ToString for a human readable output that sometimes is not so easy to parse. So yeah, I think that your ideas are kinda like correct ~~

The Repr documentation says:

A typeclass that specifies the standard way of turning values of some type into Format.
When rendered this Format should be as close as possible to something that can be parsed as the input value.

[Sofia Rodrigues]

And I think that someone needs to create a PR explaining this in the docstrings of ToString just to add some clarity.

[Kyle Miller]

Yeah, ToString is meant to be some sort of coercion to String, giving some sort of natural representation, with no concern for parsing. For example, the ToString instance for String is the identity function (it's already a string), but the Repr instance, which is supposed to be parseable, wraps it in quotation marks and transforms the string to use escape sequences when necessary.

[Violeta Hernández]

Is there a way to make #eval default to the ToString representation rather than Repr? (Is this desirable?)

[Kyle Miller]

Do #eval toString <| e instead of #eval e, if it's a pure value

[Kyle Miller]

In the next release, #eval will use the ToExpr instance over Repr or ToString. It tries each of these in order.

[Kyle Miller]

This way the result will be pretty printed just like in the Infoview.

[Violeta Hernández]

Nice!