Xpath

• Functional XSLT/XPath/XQuery - #1 Introduction 3 June 2024

  Functional XSLT/XPath/XQuery - #1 Introduction

  To the modern programmer these languages appear strange beasts, born when the world was obsessed with XML, it is declarative, almost functional, but somehow missing some of the idioms of later more consciously functional languages. For a functional programmer it misses familiar function and data types that the functional programmer instinctively expect. To be fair their roots predate much of the flowering of functional patterns in language development and whilst the semantics of the languages are quite well suited to this style of development, the foundations are idiosyncratic, and now locked in by their history to be slightly out of step.

• Functional XSLT/XPath/XQuery - #2 Functor pattern 3 June 2024

  Functional XSLT/XPath/XQuery - #2 Functor pattern

  In the previous blog we introduced some machinery, the id function, and function composition, we talked about function types, and invented an informal notion of parametric types in order to aid our thinking.

  We now move onto the functional patterns. These are a bit like OO patterns but have more formal mathematical underpinnings. Knowing what to call the things is problematic in itself, in mathematics these underpinnings are called a category, in functional circles they are often called a typeclass , this is a borrowed term from Haskell that has a particular mechanism for implementing them which may be misleading, other languages have different mechanisms, some languages have no formal mechanism, but have informal mechanisms, or no mechanism at all apart from idiomatic usage, like an OO pattern.

• Functional XSLT/XPath/XQuery - #3 Maybe data type 5 June 2024

  Functional XSLT/XPath/XQuery - #3 Maybe data type

  We are now in a position to fill in a missing data type.

  Lets cover some XPath basics.

  Consider the following XSLT

  <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                  exclude-result-prefixes="#all"
                  version="3.0"
                  xmlns:xs="http://www.w3.org/2001/XMLSchema"
                  xmlns:kooks="http://kookerella.com">
      <xsl:output method="xml" indent="yes"/>
  
      <xsl:function name="kooks:safeDivision" as="xs:numeric?">
          <xsl:param name="numerator" as="xs:numeric"/>  
          <xsl:param name="denominator" as="xs:numeric"/>
          <xsl:if test="not($denominator eq 0)">
              <xsl:sequence select="$denominator div $denominator"/>
          </xsl:if>
      </xsl:function>
  
      <xsl:template match="/">
          <output>
              <xsl:variable
                  name="calculations"
                  as="xs:numeric*"
                  select="(1 div 1,1 div 2,2 div 3)"/>
              <xsl:sequence select="$calculations"/>
          </output>
      </xsl:template>
  </xsl:stylesheet>
  

  If we execute it against an xml it returns this:

• Functional XSLT/XPath/XQuery - #4 Monoid 9 June 2024

  Functional XSLT/XPath/XQuery - #4 Monoid

  Usually in these sorts of tutorials Monoid comes first, its the simplest, easiest most familiar sort of thing, we don’t really even need code to describe it, its loosely the theory of having something you can composing two things together and getting another thing of the same type, together with an identity element, so in mathematics the integers form a monoid with + and the identity element is 0 or we can use multiplication * and 1, the set of booleans, i.e. { True , False } together with the operator AND form a monoid, where the identity element is True, to be honest, these things are rarely used as monoids in functional code, they could be, there just isn’t much use, the things that are usually used as monoids are collections, so in the XPath world this is sequence, array, map (and the recently added maybe).

• Functional XSLT/XPath/XQuery - #5 Monad pattern 15 June 2024

  Functional XSLT/XPath/XQuery - #5 Monad pattern

  The monad is probably the most written about functional pattern, it is famous for convoluted and obtuse explanations, but I will steal some of the better explanations and translate them into XSLT/XPath.

  (adapted directly from All About Monads - HaskellWiki).

  “Suppose that we are writing a program to keep track of sheep cloning experiments. We would certainly want to know the genetic history of all of our sheep, so we would need mother and father functions. But since these are cloned sheep, they may not always have both a mother and a father!