[Haskell-beginners] hGetContents and modules architecture idea

[David McBride]

In haskell you have datatypes like String, Text, Text.Lazy,
ByteString, etc.  All of those have functions like readFile,
writeFile, hPutStr, hGetLine (if applicable to that type).  If you
have your own type, say a Triangle, you would usually get that from
one of the intermediate types, such as Bytestring -> Triangle.

It is also possible to make a class which allows you to create a
Triangle from a variety of types, ToShape a => a -> Triangle, where
instance ToShape ByteString.

For your second question. To do a complex type from say a ByteString,
most people would use a parser combinator, perhaps something like
attoparsec, although there are many other options.  That particular
library allows you to parse from a bytestring or from a file as
needed.  When using it on a file you might use withFile around
parseWith and pass hGetContents as its first argument.

On Fri, May 5, 2017 at 5:31 AM, PY <aquagnu at gmail.com> wrote:
> Hello everyone! I'm trying to understand base idea of Haskell modules
> architecture. In other languages, "reading from file" is placed in something
> like "io", "istream", "file", etc modules. Also most languages have a
> concept of reading from abstract bytes streams. You read bytes from
> something and translate them into your high level object/type/etc.
>
> In Haskell I see that, for example, function hGetContents exists in  (this
> is my local installation):
>
> GHC.IO.Handle
> System.IO
> Data.ByteString
> Data.ByteString.Char8
> Data.ByteString.Lazy
> Data.ByteString.Lazy.Char8
> Data.Text.IO
> Data.Text.Lazy.IO
> System.IO.Strict
> Text.Pandoc.UTF8
> Data.ListLike
> Data.ListLike.IO
> ClassyPrelude
> Hledger.Utils.UTF8IOCompat
> Data.IOData
> Darcs.Util.Ratified
> Sound.File.Sndfile
> Sound.File.Sndfile.Buffer
> Data.String.Class
> Network.BufferType
>
> If I'll create module SuperMegaShapes with some Triangle, Rectangle, Square
> and other things, I'll create (to be consistent with Haskell-way)...
> hGetContents there??!
>
> So, I have 2 questions here:
>
> First one: let's imagine that we have Haskell compiler for embedded. And I
> want to read Text, ByteString, Sndfile and SuperMegaShapes from... SPI.
> There are many devices andprotocols, right? And I have not FILE HADNLER for
> most of them. So, this mean that Haskell (like simple script language)
> supports only concept of FILE HANDLER reading?! And no other ABSTRACTIONS?
>
> Second question is: must any new type which we plan to read/write to have
> hGetContents? What if it is packed in some tricky container? Matreshka?
> Something else, more tricky? :) And more: what other I/O functions must be
> injected in our model definitions modules?
>
>
> ===
> Best regards
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>