[Haskell-cafe] GHCi and HXT

[Daniel McAllansmith]

On Tuesday 28 November 2006 20:50, Alexis Hazell wrote:
> 	let contact = mkelem "stream:stream" [ sattr "xmlns:stream"
> "http://etherx.jabber.org/streams", sattr "xmlns" "jabber:client", sattr
> "to" "livejournal.com" ] [] :: ArrowXml a => a XmlTree XmlTree
>
> And got:
>
> 	<interactive>:1:14:
> 	Ambiguous type variable `a' in the constraint:
> 	`ArrowXml a'
> 	arising from instantiating a type signature at <interactive>:1:14-194
> 	Probable fix: add a type signature that fixes these type variable(s)
>
> i don't understand why i need to specify type signatures when they should
> already be in scope?

Is this a result of the monomorphism restriction?

> And if i /do/ need to specify the type signatures for 
> some reason, how do i do so in a manner that works?

You need to add a type signature that specifies a specific arrow data type 'a' 
that is an instance of ArrowXml.

There are many instances of ArrowXml provided in HXT, eg LA, IOSArrow, 
IOSLArrow, etc

so try something like:

let contact = mkelem "stream:stream" [ sattr "xmlns:stream" 
"http://etherx.jabber.org/streams", sattr "xmlns" "jabber:client", sattr "to" 
"livejournal.com" ] [] :: IOSArrow XmlTree XmlTree

By the way, once you have the declaration of contact in a source file, and it 
is being used by a function in which the ArrowXml instance is not ambiguous 
eg:

foo = runX contact

the type checker will be able to infer the type and you don't have to include 
the type signature.

Daniel