Explanation

Runtime model

All expressions are lazily evaluated at the time of use. If the same variable is referenced more than once, it will also be evaluated more than once. Pay attention when providing custom formatters/selectors with side effects:

counter = 0

def formatter_with_side_effect(value, locale, options):
    nonlocal counter
    counter +=1
    return str(value)

msg = Message("\
.local $foo = {|foo| :side_effect}
{{{$foo} {$foo}}}
")
msg.format(formatters={'side_effect': formatter_with_side_effect})
print(counter)  # -> 2

Function composition

Composition of functions is only allowed for the same functions:

.local $x {42 :integer}
{{x = {$x :integer}}}

This will raise an exception:

.local $x {42 :integer}
{{x = {$x :string}}}

Options are merged together with the later options overwritting any earlier options:

.local $date {|2024-05-10| :date style=short}
{{date = {$date :date style=full}}}

Whitespace

This library implements the grammar exactly as specified which includes whitespace. According to the grammar, complex messages (messages containing declarations and/or the match block) cannot have a leading whitespace. For example, this message:

    .match {$count :integer}
* {{count is {$count}}}

will be parsed as a simple message (message not containing any declarations/match blocks, only patterns {...}). The parsing will fail because a pattern cannot contain unescaped {.

When defining complex messages using multiline strings, don't forget to use a leading \ to remove the first newline character:

Message("""\  # <- notice the backslash
.match {$count: integer}
* {{count is {$count}}}""")

Alternatively, you can use this style:

Message(""".match {$count: integer}
* {{count is {$count}}}""")