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Today's Topics:
1. Re: The type class Read (mrx)
2. Re: The type class Read (Francesco Ariis)
3. Re: The type class Read (mrx)
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Message: 1
Date: Fri, 13 Jul 2018 09:06:51 +0200
From: mrx <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] The type class Read
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Den tors 12 juli 2018 13:15Francesco Ariis <[email protected]> skrev:
> Hello Patrik,
>
> On Thu, Jul 12, 2018 at 12:05:52PM +0200, mrx wrote:
> > Why is the type class called `Read`?
> > What am I missing above?
>
> we can say any instance of `Read` has to implement
>
> read :: Read a => String -> a
> -- the actual instance implements a different function,
> -- but that's not relevant for our example
>
> So, a typeclass (Read, capital `r`) gives us a function (`read`,
> lower-case `r`). The function goes from `String` (and no other
> things) to our implemented type.
>
That makes sense to me based on the type, sure. So read is some form of
casting then?
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?
// Patrik
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Message: 2
Date: Fri, 13 Jul 2018 09:30:37 +0200
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] The type class Read
Message-ID: <[email protected]>
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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.
------------------------------
Message: 3
Date: Fri, 13 Jul 2018 10:14:56 +0200
From: mrx <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] The type class Read
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Ah, I see. Thanks a lot for the clarification!
Patrik Iselind
Den fre 13 juli 2018 09:31Francesco Ariis <[email protected]> skrev:
> 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.
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