Haskell Simple XML generation gives compile error

Question

I am trying to learn to use the xmlgen library, but when I tried the example from http://factisresearch.blogspot.in/2011/05/xmlgen-feature-rich-and-high.html I get a compile error.

Here is the code:

import Text.XML.Generator
import Data.ByteString.Lazy as BSL
import Prelude as P

genXml' :: Xml Doc
genXml' =
  let people = [("Stefan", "32"), ("Judith", "4")]
  in doc defaultDocInfo $
       xelem "people" $
         xelems $ P.map (\(name, age) -> xelem "person" (xattr "age" age <#> xtext name)) people

outputXml :: IO ()
outputXml = BSL.putStr (xrender genXml')

The compiler errors:

$ ghc --make prog.hs
[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:13:25:
    No instance for (XmlOutput ByteString)
      arising from a use of `xrender'
    Possible fix:
      add an instance declaration for (XmlOutput ByteString)
    In the first argument of `BSL.putStr', namely `(xrender genXml')'
    In the expression: BSL.putStr (xrender genXml')
    In an equation for `outputXml':
        outputXml = BSL.putStr (xrender genXml')

The compiler seems to be deducing that "xrender genXML'" needs to be of type "XmlOutput ByteString" and there is an instance of this in the xmlgen library. So please help me with understanding what this error means, and how it can be fixed.

If it helps, I am using the Haskell Platform in Ubuntu 12.10, ghc 7.4.2 and the xmlgen 0.4.0.3

score 3 · Accepted Answer · answered Nov 17 '12 at 14:01

The compiler seems to be deducing that "xrender genXML'" needs to be of type "XmlOutput ByteString"

No, the compiler deduces (from the use of BSL.putStr) that xrender genXML' needs to be of type Data.ByteString.Lazy.ByteString.

Since the type of xrender is

xrender :: (Renderable r, XmlOutput t) => Xml r -> t

a necessary condition to be able to instantiate t with Data.ByteString.Lazy.ByteString is an XmlOutput instance for lazy ByteStrings.

Since there is such an instance exported from Text.XML.Generator, the only cause for the error message that I see is that your

import Data.ByteString.Lazy as BSL

imports the module from a different version of the bytestring package than the xmlgen library was built against.

Can you check that with ghc-pkg describe xmlgen, which lists the bytestring version it was built against among the dependencies, and ghc-pkg list bytestring to check which bytestring versions you have installed?

Thank you for the response. My xmlgen was built against bytestring-0.9.2.1, and I have two versions of bytestring installed: bytestring-0.9.2.1 and bytestring-0.10.0.1. If this was a proper project, I suppose I would specify the version in a .cabal file, but just for learning purposes, how can I tell the compiler to use a particular version of bytestring while compiling this file? — donatello, Nov 17 '12 at 14:23
So without specification of the version of `bytestring` to use, the compiler uses the newest. To tell it which version to use, `ghc -package bytestring-0.9.2.1 prog`. With ghc >= 7, you don't need the `--make` anymore. — Daniel Fischer, Nov 17 '12 at 14:25

Haskell Simple XML generation gives compile error

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