[Haskell-cafe] Correct parsers for bounded integral values

[Jeff Clites]

> On Aug 3, 2025, at 12:17 PM, Stefan Klinger <haskell at stefan-klinger.de> wrote:
> 
> Jeff Clites via Haskell-Cafe (2025-Jul-21, excerpt):
>> I think you will find, though, that `read @Word8` is intended to
>> match the behavior of `(fromInteger @Word8) . (read @Integer)`. At
>> least, the `Int` case is specified in the Haskell Report (in the
>> Prelude implementation).
> 
> Sorry, I cannot find this.  Would you have a URL and a line number for
> me?  I'm really sorry my google-foo seems to be insufficient.

This part is what I was thinking of:

    https://www.haskell.org/onlinereport/haskell2010/haskellch9.html#verbatim-307

That's the `Read` instance for `Int`, which defines `readsPrec` (which is used by `reads`, which is used by `read`, also defined there). It uses `fromInteger`.

This is the section of the spec with the Standard Prelude. (It says it's serving as a specification, though not requiring the actual implementation to be what's given here.)

That's only specifically about `Int`. The limited-width types are in these two sections:

    https://www.haskell.org/onlinereport/haskell2010/haskellch18.html
    https://www.haskell.org/onlinereport/haskell2010/haskellch23.html

This is the part of the spec about the standard libraries, though it doesn't show the full implementations. I don't know if they are supplied elsewhere (I've looked and can't find them), though I don't think that that means that their behavior is unspecified. It does mention, "All arithmetic is performed modulo 2^n, where n is the number of bits in the type." So it doesn't supply the `Read` implementations for these other types and it doesn't directly say that `Read` has to wrap even though arithmetic operations do. But I would expect that the intention is that they act consistently with `Int`.

When I said, "I think you will find..." in my original message, what I really meant was that if you file a bug report I think the reply will be that the behavior is as intended (specifically, to match `Int`). But who knows.

Enjoy!

Jeff