# 10. Templates: generating text

This module provides convenient subprograms for replacing specific substrings with other values. It is typically used to replace substrings like “%{version}” in a longer string with the actual version, at run time.

This module is not the same as the templates parser provided in the context of AWS, the Ada web server, where external files are parsed and processed to generate other files. The latter provides advanced features like filters, loops,…

The substrings to be replaced always start with a specific delimiter, which is set to % by default, but can be overridden in your code. The name of the substring to be replaced is then the identifier following that delimiter, with the following rules:

- If the character following the delimiter is the delimiter itself, then the final string will contain a single instance of that delimiter, and no further substitution is done for that delimiter. An example of this is “%%”.
- If the character immediately after the delimiter is a curly brace ({), then the name of the identifier is the text until the next closing curly brace. It can then contain any character expect a closing curly brace. An example of this is “%{long name}”
- If the first character after the delimiter is a digit, then the name of the identifier is the number after the delimiter. An example of this is “%12”. As a special case, if the first non-digit character is the symbol -, it is added as part of the name of the identifier, as in “%1-“. One use for this feature is to indicate you want to replace it with all the positional parameters %1%2%3%4. For instance, if you are writing the command line to spawn an external tool, to which the user can pass any number of parameter, you could specify that command line as “tool -o %1 %2-“ to indicate that all parameters should be concatenated on the command line.
- If the first character after the delimiter is a letter, the identifier follows the same rules as for Ada identifiers, and can contain any letter, digit, or underscore character. An example of this is “%ab_12”. For readability, it is recommended to use the curly brace notation when the name is complex, but that is not mandatory.
- Otherwise the name of the identifier is the single character following the delimiter

For each substring matching the rules above, the Substitute subprogram will look for possible replacement text in the following order:

- If the Substrings parameter contains an entry for that name, the corresponding value is used.
- Otherwise, if a callback was specified, it is called with the name of the identifier, and should return the appropriate substitution (or raise an exception if no such substitution makes sense).
- A default value provided in the substring itself
- When no replacement string was found, the substring is kept unmodified