[Haskell-cafe] GHCi and HXT
dm.maillists at gmail.com
Tue Nov 28 03:43:11 EST 2006
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:
> 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,
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
foo = runX contact
the type checker will be able to infer the type and you don't have to include
the type signature.
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