Re: like "deriving Read" but faster?

To highlight the point about expressions, I've found many beginners are surprised by the behavior of code like this:

    ghci> data Foo = Foo { foo :: Int }
    ghci> bar (Foo n) = n + 5
    ghci> bar Foo { foo = 9 }
    14

To point out a case where this can lead to real bugs, imagine:

    ghci> data State = State { new :: String, old :: String } deriving (Show)
    ghci> backup s = s { old = new s }
    ghci> backup (State "v1" "v0") { new = "v2" }
    State {new = "v2", old = "v2"}

Many might expect this to return (State "v2" "v1"), thinking function application would have higher precedence than the record update.

  - Tom

> On 08/18/2026 4:49 PM CEST Tikhon Jelvis <[email protected]> wrote:
> 
> 
> 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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