Why isn't ByteString converted automatically to FilePath?

Question

I'm passing a (strict) ByteString to something expecting a System.IO.FilePath, which is declared as type FilePath = String. I'm also using {-# LANGUAGE OverloadedStrings #-}. I've had conversions in some places happen automatically, but here it does not. What have I got wrong?

Main.hs:33:40: error:
    • Couldn't match type ‘ByteString’ with ‘[Char]’
      Expected type: FilePath
        Actual type: ByteString

In short, no automatic conversions happen in Haskell at all. All conversions are done explicitly. — Willem Van Onsem, Feb 22 '18 at 08:56

Willem Van Onsem · Accepted Answer · 2018-02-22T11:25:04.273

13

The {-# LANGUAGE OverloadedStrings #-} pragma only works for string literals, like "a string". In that case, Haskell implicitly places a fromString before every string literal, so it rewrites a string literal as "a string" to fromString "a string". This only happens for literals.

In Haskell, as far as I know, there are no implicit conversions. Conversions between for instance Int and Float are all explicit.

Furthermore note that the IsString typeclass only has a function fromString :: String -> a. So that means it works only from a string to that instance (here ByteString), not the other way around.

You can use the unpack :: ByteString -> String to convert the ByteString to a String.

edited Feb 22 '18 at 11:25

answered Feb 22 '18 at 08:58

Willem Van Onsem

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But do note that `unpack` is simply wrong for most encodings. And of course also beware that the meaning of a `FilePath` *also* depends on an encoding setting that's hidden behind the scenes. If you are on Linux, you can avoid a lot of headaches by simply using a file API that uses `ByteString` for the name of its files in the first place. – Daniel Wagner Feb 22 '18 at 13:37

score 4 · Answer 2 · answered Feb 22 '18 at 08:12

IIRC, the OverloadedStrings extension doesn't enable magical conversion between different types of data. What it does is that when you write a string literal like "foo", the compiler can treat that literal as not only a String, but also as a ByteString.

You probably need something like unpack to convert ByteString to String.

Why isn't ByteString converted automatically to FilePath?

2 Answers2