Question

I always run into the following error when trying to read a ByteString:
Prelude.read: no parse

Here’s a sample of code that will cause this error to occur upon rendering in a browser:

factSplice :: SnapletSplice App App
factSplice = do
    mbstr <- getParam "input" -- returns user input as bytestring
    let str = maybe (error "splice") show mbstr
    let n = read str :: Int
    return [X.TextNode $ T.pack $ show $ product [1..n]]

Or perhaps more simply:

simple bs = read (show bs) :: Int

For some reason, after show bs the resulting string includes quotes.
So in order to get around the error I have to remove the quotes then read it.
I use the following function copied from the internet to do so:

sq :: String -> String
sq s@[c]                     = s
sq ('"':s)  | last s == '"'  = init s
            | otherwise      = s
sq ('\'':s) | last s == '\'' = init s
            | otherwise      = s
sq s                         = s

Then simple bs = read (sq.show bs) :: Int works as expected.

1. Why is this the case?
2. What is the best way to convert a ByteString to an Int?

Answer 1

Show is used to create a String representation of something, that is useful for debugging and plain-text serialization. The Show typeclass is not just a fancy way of converting anything into a String. That’s why ByteString adds quotes to the string: because it’s arguably easier to read it that way when debugging or deserializing a data stream.

You can use the Data.ByteString.Char8.unpack function to convert a ByteString to a String, but note that this unpacks the ByteString byte-per-byte, which messes up high-value Unicode characters or other characters that are stored as more than one byte; if you want to do something other than using read on the result, I’d recommend converting the ByteString to Text instead, which offers more flexibility in this situation. Assuming that your encoding is UTF8 in this case (As should be the default in Snap), you can use the Data.Text.Encoding.decodeUtf8 function for this. To then convert a Text value to a String with correct Unicode symbols, you use Data.Text.unpack.

Once you have a String, you are free to read it as much as you want; alternatively, you can choose to read a Text value directly using the functions in the Data.Text.Read module.