[Haskell-cafe] How does one create an input handle bound to a string instead of a file?

John D. Ramsdell ramsdell0 at gmail.com
Thu Feb 28 16:10:45 CET 2013

The actual parser is a bit more complicated than I let on.  First,
it's important that not all of a file be read at the same time as the
files can be huge.  Second, it keeps track of column row position
information as an IORef, which makes sense because the ref is bundled
in a structure with the handle and manipulated only in the IO Monad in
conjunction with operations on the handle.


On Thu, Feb 28, 2013 at 9:00 AM, Erik Hesselink <hesselink at gmail.com> wrote:
> Is your parser impure? I would expect a function from
> String/Text/ByteString to Maybe (SExpr Pos).
>  Then you have no need for a Handle.
> Regards,
> Erik
> On Thu, Feb 28, 2013 at 2:32 PM, John D. Ramsdell <ramsdell0 at gmail.com> wrote:
>> I think I wasn't clear about my question.  I want something that
>> creates a value of type System.IO.Handle.  You see, I have a high
>> performance S-expression parser that I'd like to use in GHCi reading
>> strings while at the command loop.
>> Here is more details on my module SExpr that exports the SExpr data
>> type and the load function.  The desired function is called
>> stringHandle.
>> -- An S-expression
>> data SExpr
>>     = S String                 -- A symbol
>>     | Q String                 -- A quoted string
>>     | N Int                    -- An integer
>>     | L [SExpr a]              -- A proper list
>> -- Read one S-expression or return Nothing on EOF
>> load :: Handle -> IO (Maybe (SExpr Pos))
>> In GHCi, I want to type something like:
>> SExpr> let h = stringHandle "()"
>> SExpr> load h
>> Just (L [])
>> SExpr> load h
>> Nothing
>> SExpr>
>> It seems to me right now that I have to implement a duplicate parser
>> that implements Read.  At least S-expression parsing is easy.
>> John
>> On Thu, Feb 28, 2013 at 3:02 AM, Ganesh Sittampalam <ganesh at earth.li> wrote:
>>> Hi,
>>> On 27/02/2013 20:38, John D. Ramsdell wrote:
>>>> How does one create a value of type System.IO.Handle for reading that
>>>> takes its input from a string instead of a file?  I'm looking for the
>>>> equivalent of java.io.StringReader in Java.  Thanks in advance.
>>> http://hackage.haskell.org/package/bytestring-handle can make handles
>>> that read and write to ByteStrings.
>>> Cheers,
>>> Ganesh
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