Migrating Legacy Formats
Migrating from legacy formats (.dtd, .properties) is different from migrating Fluent to Fluent. When migrating legacy code paths, you’ll need to adjust the Fluent strings for the quirks Mozilla uses in the legacy code paths. You’ll find a number of specialized functionalities here.
Legacy Migration Tools
To assist with legacy format migrations, some scripting tools are provided:
When creating a migration, one or both of these tools may provide a good starting point for manual work by automating at least a part of the migration, including recipe generation and refactoring the calling code.
Basic Migration
Let’s consider a basic example: one string needs to be migrated, without any further change, from a DTD file to Fluent.
The legacy string is stored in toolkit/locales/en-US/chrome/global/findbar.dtd:
<!ENTITY next.tooltip "Find the next occurrence of the phrase">
The new Fluent string is stored in toolkit/locales/en-US/toolkit/main-window/findbar.ftl:
findbar-next =
.tooltiptext = Find the next occurrence of the phrase
This is how the migration recipe looks:
# Any copyright is dedicated to the Public Domain.
# http://creativecommons.org/publicdomain/zero/1.0/
from __future__ import absolute_import
import fluent.syntax.ast as FTL
from fluent.migrate.helpers import transforms_from
def migrate(ctx):
"""Bug 1411707 - Migrate the findbar XBL binding to a Custom Element, part {index}."""
ctx.add_transforms(
"toolkit/toolkit/main-window/findbar.ftl",
"toolkit/toolkit/main-window/findbar.ftl",
transforms_from(
"""
findbar-next =
.tooltiptext = { COPY(from_path, "next.tooltip") }
""", from_path="toolkit/chrome/global/findbar.dtd"))
The first important thing to notice is that the migration recipe needs file
paths relative to a localization repository, losing locales/en-US/:
toolkit/locales/en-US/chrome/global/findbar.dtdbecomestoolkit/chrome/global/findbar.dtd.
toolkit/locales/en-US/toolkit/main-window/findbar.ftlbecomestoolkit/toolkit/main-window/findbar.ftl.
The context.add_transforms function takes 3 arguments:
Path to the target l10n file.
Path to the reference (en-US) file.
An array of Transforms. Transforms are AST nodes which describe how legacy translations should be migrated.
Note
For migrations of Firefox localizations, the target and reference path are the same. This isn’t true for all projects that use Fluent, so both arguments are required.
In this case there is only one Transform that migrates the string with ID
next.tooltip from toolkit/chrome/global/findbar.dtd, and injects
it in the FTL fragment. The COPY Transform allows to copy the string
from an existing file as is, while from_path is used to avoid
repeating the same path multiple times, making the recipe more readable. Without
from_path, this could be written as:
ctx.add_transforms(
"toolkit/toolkit/main-window/findbar.ftl",
"toolkit/toolkit/main-window/findbar.ftl",
transforms_from(
"""
findbar-next =
.tooltiptext = { COPY("toolkit/chrome/global/findbar.dtd", "next.tooltip") }
"""))
This method of writing migration recipes allows to take the original FTL
strings, and simply replace the value of each message with a COPY
Transform. transforms_from takes care of converting the FTL syntax
into an array of Transforms describing how the legacy translations should be
migrated. This manner of defining migrations is only suitable to simple strings
where a copy operation is sufficient. For more complex use-cases which require
some additional logic in Python, it’s necessary to resort to the raw AST.
The example above is equivalent to the following syntax, which exposes the underlying AST structure:
ctx.add_transforms(
"toolkit/toolkit/main-window/findbar.ftl",
"toolkit/toolkit/main-window/findbar.ftl",
[
FTL.Message(
id=FTL.Identifier("findbar-next"),
attributes=[
FTL.Attribute(
id=FTL.Identifier("tooltiptext"),
value=COPY(
"toolkit/chrome/global/findbar.dtd",
"next.tooltip"
)
)
]
)
]
)
This creates a Message, taking the value from the legacy string
findbar-next. A message can have an array of attributes, each with an ID
and a value: in this case there is only one attribute, with ID tooltiptext
and value copied from the legacy string.
Notice how both the ID of the message and the ID of the attribute are
defined as an FTL.Identifier, not simply as a string.
Tip
It’s possible to concatenate arrays of Transforms defined manually, like in
the last example, with those coming from transforms_from, by using
the + operator. Alternatively, it’s possible to use multiple
add_transforms.
The order of Transforms provided in the recipe is not relevant, the reference file is used for ordering messages.
Replacing Content in Legacy Strings
While COPY allows to copy a legacy string as is, REPLACE
(from fluent.migrate) allows to replace content while performing the
migration. This is necessary, for example, when migrating strings that include
placeholders or entities that need to be replaced to adapt to Fluent syntax.
Consider for example the following string:
<!ENTITY aboutSupport.featuresTitle "&brandShortName; Features">
Which needs to be migrated to:
features-title = { -brand-short-name } Features
The entity &brandShortName; needs to be replaced with a term reference:
FTL.Message(
id=FTL.Identifier("features-title"),
value=REPLACE(
"toolkit/chrome/global/aboutSupport.dtd",
"aboutSupport.featuresTitle",
{
"&brandShortName;": TERM_REFERENCE("brand-short-name"),
},
)
),
This creates an FTL.Message, taking the value from the legacy string
aboutSupport.featuresTitle, but replacing the specified text with a
Fluent term reference.
Note
REPLACE replaces all occurrences of the specified text.
It’s also possible to replace content with a specific text: in that case, it
needs to be defined as a TextElement. For example, to replace
example.com with HTML markup:
value=REPLACE(
"browser/chrome/browser/preferences/preferences.properties",
"searchResults.sorryMessageWin",
{
"example.com": FTL.TextElement('<span data-l10n-name="example"></span>')
}
)
The situation is more complex when a migration recipe needs to replace
printf arguments like %S. In fact, the format used for localized
and source strings doesn’t need to match, and the two following strings using
unordered and ordered argument are perfectly equivalent:
btn-quit = Quit %S
btn-quit = Quit %1$S
In this scenario, replacing %S would work on the first version, but not
on the second, and there’s no guarantee that the localized string uses the
same format as the source string.
Consider also the following string that uses %S for two different
variables, implicitly relying on the order in which the arguments appear:
updateFullName = %S (%S)
And the target Fluent string:
update-full-name = { $name } ({ $buildID })
As indicated, REPLACE would replace all occurrences of %S, so
only one variable could be set. The string needs to be normalized and treated
like:
updateFullName = %1$S (%2$S)
This can be obtained by calling REPLACE with
normalize_printf=True:
FTL.Message(
id=FTL.Identifier("update-full-name"),
value=REPLACE(
"toolkit/chrome/mozapps/update/updates.properties",
"updateFullName",
{
"%1$S": VARIABLE_REFERENCE("name"),
"%2$S": VARIABLE_REFERENCE("buildID"),
},
normalize_printf=True
)
)
Attention
To avoid any issues normalize_printf=True should always be used when
replacing printf arguments. This is the default behaviour when working
with .properties files.
Note
VARIABLE_REFERENCE, MESSAGE_REFERENCE, and
TERM_REFERENCE are helper Transforms which can be used to save
keystrokes in common cases where using the raw AST is too verbose.
VARIABLE_REFERENCE is used to create a reference to a variable, e.g.
{ $variable }.
MESSAGE_REFERENCE is used to create a reference to another message,
e.g. { another-string }.
TERM_REFERENCE is used to create a reference to a term,
e.g. { -brand-short-name }.
Both Transforms need to be imported at the beginning of the recipe, e.g.
from fluent.migrate.helpers import VARIABLE_REFERENCE
Trimming Unnecessary Whitespaces in Translations
Note
This section was updated in May 2020 to reflect the change to the default
behavior: legacy translations are now trimmed, unless the trim
parameter is set explicitly.
It’s not uncommon to have strings with unnecessary leading or trailing spaces in legacy translations. These are not meaningful, don’t have practical results on the way the string is displayed in products, and are added mostly for formatting reasons. For example, consider this DTD string:
<!ENTITY aboutAbout.note "This is a list of “about” pages for your convenience.<br/>
Some of them might be confusing. Some are for diagnostic purposes only.<br/>
And some are omitted because they require query strings.">
By default, the COPY, REPLACE, and PLURALS
transforms will strip the leading and trailing whitespace from each line of the
translation, as well as the empty leading and trailing lines. The above string
will be migrated as the following Fluent message, despite copious indentation
on the second and the third line in the original:
about-about-note =
This is a list of “about” pages for your convenience.<br/>
Some of them might be confusing. Some are for diagnostic purposes only.<br/>
And some are omitted because they require query strings.
To disable the default trimming behavior, set trim:"False" or
trim=False, depending on the context:
transforms_from(
"""
about-about-note = { COPY("toolkit/chrome/global/aboutAbout.dtd", "aboutAbout.note", trim:"False") }
""")
FTL.Message(
id=FTL.Identifier("discover-description"),
value=REPLACE(
"toolkit/chrome/mozapps/extensions/extensions.dtd",
"discover.description2",
{
"&brandShortName;": TERM_REFERENCE("-brand-short-name")
},
trim=False
)
),
Concatenating Strings
It’s best practice to only expose complete phrases to localization, and to avoid
stitching localized strings together in code. With DTD and properties,
there were few options. So when migrating to Fluent, you’ll find
it quite common to concatenate multiple strings coming from DTD and
properties, for example to create sentences with HTML markup. It’s possible to
concatenate strings and text elements in a migration recipe using the
CONCAT Transform.
Note that in case of simple migrations using transforms_from, the
concatenation is carried out implicitly by using the Fluent syntax interleaved
with COPY() transform calls to define the migration recipe.
Consider the following example:
# %S is replaced by a link, using searchResults.needHelpSupportLink as text
searchResults.needHelp = Need help? Visit %S
# %S is replaced by "Firefox"
searchResults.needHelpSupportLink = %S Support
In Fluent:
search-results-need-help-support-link = Need help? Visit <a data-l10n-name="url">{ -brand-short-name } Support</a>
This is quite a complex migration: it requires to take 2 legacy strings, and concatenate their values with HTML markup. Here’s how the Transform is defined:
FTL.Message(
id=FTL.Identifier("search-results-help-link"),
value=REPLACE(
"browser/chrome/browser/preferences/preferences.properties",
"searchResults.needHelp",
{
"%S": CONCAT(
FTL.TextElement('<a data-l10n-name="url">'),
REPLACE(
"browser/chrome/browser/preferences/preferences.properties",
"searchResults.needHelpSupportLink",
{
"%1$S": TERM_REFERENCE("brand-short-name"),
},
normalize_printf=True
),
FTL.TextElement("</a>")
)
}
)
),
%S in searchResults.needHelpSupportLink is replaced by a reference
to the term -brand-short-name, migrating from %S Support to {
-brand-short-name } Support. The result of this operation is then inserted
between two text elements to create the anchor markup. The resulting text is
finally used to replace %S in searchResults.needHelp, and used as
value for the FTL message.
Important
When concatenating existing strings, avoid introducing changes to the original text, for example adding spaces or punctuation. Each language has its own rules, and this might result in poor migrated strings. In case of doubt, always ask for feedback.
When more than 1 element is passed in to concatenate, CONCAT
disables whitespace trimming described in the section above on all legacy
Transforms passed into it: COPY, REPLACE, and
PLURALS, unless the trim parameters has been set
explicitly on them. This helps ensure that spaces around segments are not
lost during the concatenation.
When only a single element is passed into CONCAT, however, the
trimming behavior is not altered, and follows the rules described in the
previous section. This is meant to make CONCAT(COPY()) equivalent
to a bare COPY().
Plural Strings
Migrating plural strings from .properties files usually involves two
Transforms from fluent.migrate.transforms: the
REPLACE_IN_TEXT Transform takes TextElements as input, making it
possible to pass it as the foreach function of the PLURALS Transform.
Consider the following legacy string:
# LOCALIZATION NOTE (disableContainersOkButton): Semi-colon list of plural forms.
# See: http://developer.mozilla.org/en/docs/Localization_and_Plurals
# #1 is the number of container tabs
disableContainersOkButton = Close #1 Container Tab;Close #1 Container Tabs
In Fluent:
containers-disable-alert-ok-button =
{ $tabCount ->
[one] Close { $tabCount } Container Tab
*[other] Close { $tabCount } Container Tabs
}
This is how the Transform for this string is defined:
FTL.Message(
id=FTL.Identifier("containers-disable-alert-ok-button"),
value=PLURALS(
"browser/chrome/browser/preferences/preferences.properties",
"disableContainersOkButton",
VARIABLE_REFERENCE("tabCount"),
lambda text: REPLACE_IN_TEXT(
text,
{
"#1": VARIABLE_REFERENCE("tabCount")
}
)
)
)
The PLURALS Transform will take care of creating the correct number of plural
categories for each language. Notice how #1 is replaced for each of these
variants with { $tabCount }, using REPLACE_IN_TEXT and
VARIABLE_REFERENCE("tabCount").
In this case it’s not possible to use REPLACE because it takes a file
path and a message ID as arguments, whereas here the recipe needs to operate on
regular text. The replacement is performed on each plural form of the original
string, where plural forms are separated by a semicolon.
Explicit Variants
Explicitly creating variants of a string is useful for platform-dependent terminology, but also in cases where you want a one-vs-many split of a string. It’s always possible to migrate strings by manually creating the underlying AST structure. Consider the following complex Fluent string:
use-current-pages =
.label =
{ $tabCount ->
[1] Use Current Page
*[other] Use Current Pages
}
.accesskey = C
The migration for this string is quite complex: the label attribute is
created from 2 different legacy strings, and it’s not a proper plural form.
Notice how the first string is associated to the 1 case, not the one
category used in plural forms. For these reasons, it’s not possible to use
PLURALS, the Transform needs to be crafted recreating the AST.
FTL.Message(
id=FTL.Identifier("use-current-pages"),
attributes=[
FTL.Attribute(
id=FTL.Identifier("label"),
value=FTL.Pattern(
elements=[
FTL.Placeable(
expression=FTL.SelectExpression(
selector=VARIABLE_REFERENCE("tabCount"),
variants=[
FTL.Variant(
key=FTL.NumberLiteral("1"),
default=False,
value=COPY(
"browser/chrome/browser/preferences/main.dtd",
"useCurrentPage.label",
)
),
FTL.Variant(
key=FTL.Identifier("other"),
default=True,
value=COPY(
"browser/chrome/browser/preferences/main.dtd",
"useMultiple.label",
)
)
]
)
)
]
)
),
FTL.Attribute(
id=FTL.Identifier("accesskey"),
value=COPY(
"browser/chrome/browser/preferences/main.dtd",
"useCurrentPage.accesskey",
)
),
],
),
This Transform uses several concepts already described in this document. Notable
is the SelectExpression inside a Placeable, with an array
of Variant objects. Exactly one of those variants needs to have
default=True.
This example can still use transforms_from()`(), since existing strings
are copied without interpolation.
transforms_from(
"""
use-current-pages =
.label =
{ $tabCount ->
[1] { COPY(main_dtd, "useCurrentPage.label") }
*[other] { COPY(main_dtd, "useMultiple.label") }
}
.accesskey = { COPY(main_dtd, "useCurrentPage.accesskey") }
""", main_dtd="browser/chrome/browser/preferences/main.dtd"
)