I try to write a simple function to provide me with a random letter each time i call it but I have difficulties combining my idea with the concept of a functional programing approach. Some help along the way would be appreciated! The code I have looks like:
<xd:doc>
<xd:desc>Provides one random letter, if the type is provided it returns a letter of thet type</xd:desc>
<xd:param name="type">The type of letter to return, one of (A,a,B,b)</xd:param>
</xd:doc>
<xsl:function name="gdpr:randomLetter" as="xs:string">
<xsl:param name="type" as="xs:string"></xsl:param>
<xsl:choose>
<xsl:when test="$type = 'A'">
<xsl:variable name="randomNumber" select="random-number-generator()['next']?permute(1 to 7)[1]"/>
<xsl:variable name="letters" select="('A','O','U','E','I','Y','Q')"/>
<xsl:value-of select="$letters[$randomNumber]"/>
</xsl:when>
<xsl:when test="$type = 'a'">
<xsl:variable name="randomNumber" select="random-number-generator()['next']?permute(1 to 7)[1]"/>
<xsl:variable name="letters" select="('a','o','u','e','i','y','q')"/>
<xsl:value-of select="$letters[$randomNumber]"/>
</xsl:when>
<xsl:when test="$type = 'B'">
<xsl:variable name="randomNumber" select="random-number-generator()['next']?permute(1 to 19)[1]"/>
<xsl:variable name="letters" select="('W','R','T','P','S','D','F','G','H','J','K','L','M','N','B','V','C','X','Z')"/>
<xsl:value-of select="$letters[$randomNumber]"/>
</xsl:when>
<xsl:when test="$type = 'b'">
<xsl:variable name="randomNumber" select="random-number-generator()['next']?permute(1 to 19)[1]"/>
<xsl:variable name="letters" select="('w','r','t','p','s','d','f','g','h','j','k','l','m','n','b','v','c','x','z')"/>
<xsl:value-of select="$letters[$randomNumber]"/>
</xsl:when>
<xsl:otherwise>
<xsl:variable name="randomNumber" select="random-number-generator()['next']?permute(1 to 52)[1]"/>
<xsl:variable name="letters" select="('A','O','U','E','I','Y','Q','a','o','u','e','i','y','q','w','r','t','p','s','d','f','g','h','j','k','l','m','n','b','v','c','x','z','W','R','T','P','S','D','F','G','H','J','K','L','M','N','B','V','C','X','Z')"/>
<xsl:value-of select="$letters[$randomNumber]"/>
</xsl:otherwise>
</xsl:choose>
</xsl:function>
Your question encapsulates the problem:
I try to write a simple function to provide me with a random letter each time i call it
But a function that produces different results on different invocations (with the same arguments) is not a true ("pure") function.
One way out of this is to exploit the fact that XSLT already has "impure" functions of a kind: a function that creates a new node returns a different node each time, and you can expose this by using generate-id(). So you could write
<xsl:function name="my:random" as="xs:double">
<xsl:variable name="dummy"><a/></xsl:variable>
<xsl:sequence select="fn:random-number-generator(generate-id($dummy))?permute(1 to 10)"/>
</xsl:function>
The only problem with this is that you're right on the boundaries of what's well-defined in the spec and the optimizer might not let you get away with such tricks. It's much better, if you can, to find some way of passing a different argument to the function each time it is called: for example, a sequence number, or generate-id() applied to the input node you are currently processing.
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