Proposal: Derive Read instance for Scheme type

[nattybear]

Background

Currently, the Scheme type in this package derives Show, but not Read.
As a result, it’s not possible to parse a Scheme value directly from a string using the standard Read interface (e.g. read "Http" :: Scheme).

Proposal

I propose adding a deriving (Read) clause to the Scheme data type, so that users can easily reconstruct Scheme values from textual representations.

Example:

data Scheme = Http | Https
  deriving (Eq, Show, Read)

Rationale

• Improves consistency with other simple enumerations that derive both Show and Read.
• Enables convenient usage in debugging, testing, and serialization scenarios.
• The generated instance will match the existing constructor names ("Http", "Https"), which keeps it predictable.

Alternatives

• Keep as-is and let users define their own Read instance if needed.
• Provide a custom parser instead of a Read instance.

Questions for Discussion

• Would adding deriving (Read) be acceptable given backward compatibility expectations?
• Should the Read instance be case-sensitive (default) or should we consider a custom, case-insensitive one?

---

If there’s consensus, I can prepare a small PR adding the Read derivation.

[tchoutri]

@nattybear Hi, I hear what you say about being able to use the standard Read typeclass. I understand it's very convenient, but unfortunately it is one of the most shameful pieces of standard Haskell that exists. The generated Read instances are usually very inefficient, and it does not replace proper parsing with a parser library like megaparsec.

Read is the complementary to Show, which are to be used for debugging and testing (reading & writing data structures from and to Haskell code). For user-facing parsing & printing, I would strongly advise using megaparsec and text-display.

To conclude: Yes I am open a PR that would add an automatically-derived instance of Read, but nothing that would make it semi-appealing to use instead of a proper parser library. :)

[tchoutri]

fixed by #1848