MidPoint Expression Language Troubleshooting

Last modified 13 Mar 2026 16:18 +01:00
MidPoint Expression Language feature
This page describes configuration of MidPoint Expression Language midPoint feature. Please see the feature page for more details.
Table of Contents

Common Errors

Expression written in midPoint expression language (MEL) are interpreted by common expression language (CEL) interpreter. CEL is a foundational language on which MEL expressions are built. However, the CEL compiler and runtime which is used to evaluate MEL expressions is very technical in nature. While we have made a lot of convenience improvements and extensions while creating MEL, we could not fundamentally change the nature of underlying CEL engine.

The nature of CEL engine manifests itself mostly in cryptic and often confusing error messages. Below is a list mysterious of error messages that are likely to occur when constructing MEL expressions, together with explanation and tips on problem resolution.

Field selections must be performed on messages or maps.

This error message probably indicates that a null value was used as a base for structured data resolution. E.g. for code input.orig, where variable input is null. The solution is to use .? operator for optional resolution, e.g. input.?orig.

Found no matching overload for '?:_' applied to …​ (candidates: (bool, %A8, %A8))

Conditional operator (?:) must have the same data type in both branches. E.g. both branches must evaluate to string. This error is usually raised when one of the branches evaluates to string, while the other evaluates to polystring. While strings and polystrings are functionally equivalent in most cases, conditional operator is an exception. The solution is to explicitly convert the polystring branch to string using str() or stringify() function, e.g.:

isPresent(givenName) ? '%s %s'.format(givenName, familyName) : str(familyName)

Even better solution would be to replace the conditional with default() or format.concatName() function.

No matching overload for function '+'. Overload candidates: add_string

This error indicates that the add operator (`) was invoked with an unexpected operand. Most likely situation is that one of the operands is `null`. This error often happens when addition operator (`) is used for string concatenation of optional values, e.g.:

'This is ' + input            // This code is fragile

The expression above fails if the input variable is null, producing the error message above. The code can be made more robust by using the format() function:

'This is %s'.format(input)    // Robust code

However, even such code may not be perfect, as the expression above would produce string This is null in case that the input variable is null. This is usually not the desired output. The expression can further be improved by using default() function:

'This is %s'.format(default(input,'nothing'))  // Correct code

Such expression would produce string This is nothing in case that the input variable is null, which is probably more acceptable output.

Null Value Checks

Does your expression contain a check for null value which does not seem to work?

To cut a very long story short: Never use equality operator to check for null:

Wrong
foo == null

Use isNull() or isPresent() function instead, and all will be fine.

Correct
isNull(foo)

These problems are caused by CEL engine using two different ways to represent value which is not present (null and emtpty optional). Detailed explanation of this phenomenon is provided in See Null and Optional Values section of MEL specification.

Common Problems

My "foo" string does not work

Strings are delimited by apostrophes in MEL. Correct string literal is 'foo'.

Check for isPresent(focus.fullName) fails.

Of course it does. If focus does not have fullName property, then the code focus.fullName fails, and the isPresent() function is not executed. The correct form is using optional selection (.?) to avoid failing on missing property: isPresent(focus.?fullName). Alternatively, CEL has() macro could be used: has(focus.fullName). However, the has() macro fails in case that the focus variable is null. The use of isPresent(focus.?fullName) is recommended.

Variable user is not there!

That is right. Variable user is deprecated for many years, and it is not exposed to new expression environments such as MEL. Use variables such as focus and projection instead.

String formatting: '%s %s'.format(foo, bar) does not work.

CEL does not support functions with variable number of arguments (vararg). Therefore you have to use list literal ([]) when passing multiple arguments to logging functions: '%s %s'.format([ foo, bar ])

Logging: log.error('{} - {}', foo, bar) does not work.

CEL does not support functions with variable number of arguments (vararg). Therefore you have to use list literal ([]) when passing multiple arguments to logging functions: log.error('{} - {}', [ foo, bar ])

See Also

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