meson/docs/markdown/snippets/fs_read.md

Support for reading files at configuration time with the fs module

Reading text files during configuration is now supported. This can be done at any time after project has been called

project(myproject', 'c')
license_text = run_command(
    find_program('python3'), '-c', 'print(open("COPYING").read())'
).stdout().strip()
about_header = configuration_data()
about_header.add('COPYRIGHT', license_text)
about_header.add('ABOUT_STRING', meson.project_name())
...

There are several problems with the above approach:

1. It's ugly and confusing
2. If COPYING changes after configuration, Meson won't correctly rebuild when configuration data is based on the data in COPYING
3. It has extra overhead

fs.read replaces the above idiom thus:

project(myproject', 'c')
fs = import('fs')
license_text = fs.read('COPYING').strip()
about_header = configuration_data()
about_header.add('COPYRIGHT', license_text)
about_header.add('ABOUT_STRING', meson.project_name())
...

They are not equivalent, though. Files read with fs.read create a configuration dependency on the file, and so if the COPYING file is modified, Meson will automatically reconfigure, guaranteeing the build is consistent. It can be used for any properly encoded text files. It supports specification of non utf-8 encodings too, so if you're stuck with text files in a different encoding, it can be passed as an argument. See the meson object documentation for details.