Why Frege doesn't provide a read function

[zhou...@163.com]

I want to read a simple literal [Byte] into Frege, to my surprised there is no read available

[Ingo W.]

Right.

The whole Read class is missing/incomplete.

[Marimuthu Madasamy]

As a workaround, you can, of course, use the native implementation:

pure native parseByte java.lang.Byte.parseByte :: String -> (NumberFormatException | Byte)

[zhou...@163.com]

Not a bad thing, instead I use Frege prelude NanoParsec library and quickly build a parser and it is fantasy

[zhou...@163.com]

In fact, I use NanoParsec and now it is flexible enough to parse things like "apdu #x3F #xbE #xEf", I include the whole source, hope it helps

import froid.util.Log

import frege.data.Char hiding (isNumber)

import frege.data.NanoParsec

import Control.Concurrent as C

import io.github.mchav.tryfrege.Utilities

data Command = Apdu [Byte]

             | Reset

             | Finish

             | Exception

derive Show Command

hexdigit    = satisfy (Char.isHexDigit   :: Char -> Bool)

h1 :: Parser [] Char [Char]

h1 = do

  a <- expect '#'

  b <- expect 'x'

  c <- hexdigit

  d <- hexdigit

  return $ [c, d]

h2 :: Parser [] Char [[Char]]

h2 = do

  spaces *> h1 `sepBy` spaces

h3 :: Parser [] Char [[Char]]

h3 = do

  a <- spaces *> some letter <* spaces

  b <- h2

  return $ a:b

h4 :: Parser [] Char [[Char]]

h4 = do

  spaces *> h3 <* spaces

parseDriver :: Parser [] Char [[Char]] -> ([] Char) -> IO [[Char]]

parseDriver p s = case runid p s of

    (Left msg, str) -> do

      error $ "Parse failed!" ++ (reporterror str msg)

    (Right r, str) -> do

        if (null str) then return r

          else error $ "Parse failed: " ++ (reporterror str "Tokens left")

digitToByte = intToByte . digitToInt

parseJobs input = do

  r <- parseDriver h4 (unpacked input)

  case r of

    ['a','p','d','u']:d -> return $ Apdu $ map (\[a,b]-> (digitToByte a) * 16 + digitToByte b) d

    [['r','e','s','e','t']] -> return Reset

    [['f','i','n','i','s','h']] -> return Finish

    [['e','x','c','e','p','t','i','o','n']] -> return Exception

    _ -> return Exception

在 2018年1月28日星期日 UTC+8上午4:47:17，Marimuthu Madasamy写道：

[Tony Morris]

I concur that the Read type-class is terrible and can always be replaced with an improvement. It was deleted from our default Haskell prelude 10 years, and not a single time have we missed it.

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