[Haskell-cafe] Correct parsers for bounded integral values

[Pierre Thierry]

Le 21/07/2025 à 17:42, Stefan Klinger a écrit :
>      x = 123456
> is equivalent to
>      x = fromInteger (read "123456" :: Integer)

It's basically what happens internally. You can see the convertInt 
<https://hackage.haskell.org/package/ghc-internal-9.1201.0/docs/src/GHC.Internal.Read.html#convertInt> 
function and it's used a few lines beneath in the Read Int instance.

convertInt :: Num a => L.Lexeme -> ReadPrec a
convertInt (L.Number n)
  | Just i <- L.numberToInteger n = return (fromInteger i)

> But there is a *different* read function
>
>      read :: String -> Word8
>
> which is not used in the scenario above, and this is the buggy one
> (and all its bounded cousins).
Well, IIUC, it conforms to the Haskell 98 specification. In that sense, 
it's not buggy. But the specification may be sound and counter-intuitive 
(which is not an uncommon combination…).

Curiously,
Pierre Thierry
-- 

pierre at nothos.net
0xD9D50D8A

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