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

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. John On Thu, Feb 28, 2013 at 9:00 AM, Erik Hesselink 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 wrote:

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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 wrote:

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Hi,

On 27/02/2013 20:38, John D. Ramsdell wrote:

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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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