How to check if a variable is not defined?

[ilg-ul]

For user friendliness reasons, I would like to check if a variable is not defined and issue a proper error message instead of the current ERROR: Unknown variable.

Is this possible?

[eli-schwartz]

What is improper about the error message that meson produces? What error message do you want to give instead?

Note that get_variable('varname', 'fallback value') exists.

[ilg-ul]

What error message do you want to give instead?

The project uses multiple configuration files, the definitions are expected to be in one file, written by the user, and the error is issued from a different one, included from something like a library.

The similar parallel CMake configuration in the same project reads:

if(NOT DEFINED xpack_platform_compile_definition)
  message(FATAL_ERROR "Define xpack_platform_compile_definition in platform-native/cmake/dependencies.cmake")
endif()

'fallback value'

There are no fallback values.

[mitchgrout]

https://mesonbuild.com/Reference-manual_functions.html#is_variable

A one-to-one conversion from cmake to meson would be

if not is_variable('xpack_platform_compile_defintion')
  error('Define xpack_platform_compile_definition in <...>')
endif

however I would probably pick a more natural expression like

assert(is_variable('xpack_platfrm_compile_definition', ...), 'diagnostic message goes here')

---

There are other options on how to deal with this depending on how you're getting these values; if you could provide a simplified project structure, we could discuss how best to set this up?

[ilg-ul]

Thank you, @mitchgrout, that was helpful.