[Haskell-beginners] The type class Read

[Francesco Ariis]

On Fri, Jul 13, 2018 at 09:06:51AM +0200, mrx wrote:
> That makes sense to me based on the type, sure. So read is some form of
> casting then?

Yep, but just from `String` and nothing else.

> 
> Does this answers your question?
> 
> 
> Maybe, but I still don't see what I'd use it for. Is it used to for example
> read the contents of a file whose file name is provided as that string?

No, you would use `readFile` for that:

    readFile :: FilePath -> IO String
    -- Filepath is a type synonym for `String`

You would use `read` to convert simple user input (which is usually
collected as String) into, say, Integers

    getLine :: IO String
    -- this could need read

And in general, `Read` is supposed to be compatible with `Show`, so
if you used `show` for any reason (some form of cheap serialisation,
etc.), `read` should work back the type:

    λ> show [1..10]
    "[1,2,3,4,5,6,7,8,9,10]"
    λ> read it :: [Int]
    [1,2,3,4,5,6,7,8,9,10]

tl;dr: cheap type parsing. For any more specialised/complex parsing,
use a proper parsing library like Parsec.