Validating xml parser definition

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A transaction consists of an account, the type of transaction (either a deposit or a withdrawal), and the amount. For use cases where errors must be handled, we normally want to provide special handling for errors - for instance, logging them so the correct data can be entered, and the source of the errors corrected.

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We'll just print the Haskell data structures in the document so we can verify that the XML was processed correctly. Passing the XML document instance created by to invoke an IO function as part of an arrow.

We're going to be using the Haskell XML library , which are very powerful, but unfortunately very complicated. Since we want run either the error handler, or the standard processing, we need to convert processing - printing the transactions - to an arrow.

If it's handling medical data - or worse yet, controlling medical equipment - then those errors could turn into a loss of life. I suggest you play with the above two examples, feeding them various errors to see how the two handle the different cases. Text (Text, pack) type Comment = Maybe Text data Transaction = Deposit ! This is documented on the Haskell wiki Discuss this tutorial in the FP Complete Google Community.

XML provides tools for dealing with such conditions, and this tutorial is going to explore some of those. It will process transactions made at a single ATM during the course of the day. Fixed (Centi) type Comment = Maybe Text data Transaction = Deposit ! Errors that make the XML ill-formed as well as invalid, and errors in the data - non-numeric - are all worth investigating.

uses the non-validating parser by default, but it can also activate validation.

Activating validation allows the application to tell whether the XML document contains the right tags or whether those tags are in the right sequence.

For each tag, they list the valid attributes, contents and types for the tag.

An XML document is said to be well-formed if it is syntactically correct XML, and the open and close tags pair up properly.

A program misconfiguration can cause no end of problems, over and above missing data. While we could catch this case - and others - by adding code similar to the handling of comments to every arrow in , that would result in noticably longer, more opaque code. You might verify that you get that behavior with all the various errors you tried above.

If your application is processing financial data, that error could turn into a loss of money. The ad hoc nature of such additions would lead to more errors and more fragile code. If you're converting to/from XML, then the toolbox includes tools for building pickler/unpickler pairs for converting to and from XML with about as much code as it takes to write one of the pair.


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