Re: like "deriving Read" but faster?

Just Foo { foo = 42 } is valid Haskell syntax. I actually take advantage of
this in patterns sometimes, although it can definitely be a bit confusing.
(And in expressions, I 100% would have extra parentheses, otherwise it's
too visually confusing.)

On Tue, Aug 18, 2026 at 7:45 AM Johannes Waldmann via Haskell-Cafe <
[email protected]> wrote:

> Dear Cafe,
>
> I could use a parser that is equivalent
> to the one produced by "deriving Read" but ...
>
> A) faster, and using less space (Data.Text(.Lazy) instead of String)
> B) streaming (parse a long list literal "[0,1,2, ... ]" in constant space,
>    produce the first cons of the result right after reading "[0")
> C) with some minimal error reporting (e.g., some prefix of not-consumed
> input)
>
> perhaps attoparsec with some form of early-commited choice?
>
> I do have something (only for A) at
> https://codeberg.org/jwaldmann/text-read  (attoparsec on lazy text)
> and it seems twice as fast as "deriving Read" in my tests.
>
>
> NB: I was quite shocked to see that "deriving Read" accepts
> "Just Foo { foo = 42 }" (no parentheses around the record literal)
> (while "deriving Show" produces them).
>
> - J.W.
>
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