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Knowledgebase:
xdmp:value, xdmp:eval, and inline functions
11 December 2019 04:32 PM

xdmp:value() vs. xdmp:eval():

Both xdmp:value() and xdmp:eval() are used for executing strings of code dynamically. However, there are fundamental difference between the two:

  • The code in the xdmp:value() is evaluated against the current context - if variables are defined in the current scope, they may be referenced without re-declaring them
  • xdmp:eval() creates an entirely new context that has no knowledge of the context calling it - which means one must define a new XQuery prolog and variables from the main context. Those newly defined variables are then passed to the xdmp:eval() call as parameters and declared as external variables in the eval script

Function behavior when used inline:

Although both these functions seem to fulfill the same purpose, it is very important to note their behaviors changes when used inline. Consider the following example:

declare namespace db = “http://marklogic.com/xdmp/database”;
Let $t:= <database xmlns=”http://marklogic.com/xdmp/database” xmlns:xsi=”http://www.w3.org/2001/XMLSchema-instance”>
<database-name>aptp-dev-modules</database-name>
              </database>

return

fn:fold-left(function($a, $b){ xdmp:value(fn:concat(“$t/db:”, “database-name”)) }, (), (1,2,3))
(or)
fn:fold-left(function($a, $b){ $t/xdmp:value(fn:concat(“db:”, “database-name”)) }, (), (1,2,3))

When a function is called inline, the expressions inside the function cannot be statically compiled to function items because the values of the closed-over variables are not yet available. Therefore, the query parser would have to look for any variable bindings during dynamic analysis to be able to evaluate the expression. Ideally, variables from the main context are passed to the function call as parameters. However, in the case of xdmp:value(), the function is expected to have the needed context to evaluate the expression and therefore the expression is evaluated without looking for any variable bindings - which can ultimately lead to unexpected behavior. This explains why the first return statement in the above example returns an ‘empty sequence’ and the second one returns the correct results because the variable is being referenced outside of the xdmp:value call. In other words, when used inline - xdmp:value() cannot reference variables declared in the current scope.

In contrast, in the case of xdmp:eval, the parser would know to look for variable bindings during dynamic analysis as this function is not expected to have the knowledge of the calling context. Consequently, when using xdmp:eval the context needs to be explicitly created and the variables explicitly passed to the call as parameters and declared as external variables.

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