Xml parse error with empty element of type xsd:string

mlabs-haskell / opc-xml-da-client

OPC XML-DA Client for Haskell

MIT License

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Xml parse error with empty element of type xsd:string #17

Closed olafklinke closed 2 years ago

olafklinke commented 2 years ago

The following should return the empty text but fails.

{-# LANGUAGE OverloadedStrings #-}
import OpcXmlDaClient.Protocol.XmlParsing
import XmlParser
import Data.ByteString (ByteString)
import Data.Text (Text)
emptyString = "<Value xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" xsi:type=\"xsd:string\"></Value>" :: ByteString
parseByteString value emptyString -- fails with "no nodes left"

The reason is that for xsd:string the definition of value is essentially this:

textElt = children $ contentNode textContent :: Element Text

but emptyString has no children. Insert any text in the Value tag and the parser succeeds.

blamario commented 2 years ago

I don't see anything wrong with this behaviour as far as XML goes, but I'm not familiar with the OPC spec. Can you point to the section of the spec that explains this?

olafklinke commented 2 years ago

The OPC spec has nothing to do with this. Attached is part of a BrowseResponse from an OPC server which is not parsed by opc-xml-da-client because a Value element with type annotation xsd:string (thus a general xml string type) and no content is not parsed.

<Properties
                        xsi:type="ns1:ItemProperty"
                        Description="Item value"
                        ItemName="Alias"
                        Name="ns1:value"
                        ItemPath="Loc/Wec/Plant1/">
                        <Value
                            xsi:type="xsd:string">
                            </Value>
</Properties>

olafklinke commented 2 years ago

This is linked to an issue in xml-parser which when resolved can help implement a fix.