How can I save a variable as a bytestring?

August 12, 2022

Ik this is a dumb question, but if I have this:

a :: B.ByteString
a = "a"

I get an error that says "Couldn’t match type B.ByteString with type [Char]". I know what’s the problem but I don’t know how to fix it, could you help? thx.

>Solution :

Character string literals in Haskell, by default, are always treated as String, which is equivalent to [Char]. Most string-like data structures define a function called pack to convert from, and the bytestring package is no exception (Note that this is pack from Data.ByteString.Char8; the one in Data.ByteString converts from [Word8]).

import Data.ByteString.Char8(pack)

a :: B.ByteString
a = pack "a"

However, GHC also supports an extension called OverloadedStrings. If you’re willing to enable this, ByteString implements a typeclass called IsString. With this extension enabled, the type of a string literal like "a" is no longer [Char] and is instead forall a. IsString a => a (similar to how the type of numerical literals like 3 is forall a. Num a => a). This will happily specialize to ByteString if the type is in scope.

{-# LANGUAGE OverloadedStrings #-}

a :: B.ByteString
a = "a"

If you go this route, make sure you understand the proviso listed in the docs for this instance. For ASCII characters, it won’t pose a problem, but if your string has Unicode characters outside the ASCII range, you need to be aware of it.