Generating Code#

A "code writer" is the main abstraction used to generate code. It can be used to write basically any kind of code, including whitespace sensitive and brace-based. The following example generates some Python code:

SimpleCodeWriter writer = new SimpleCodeWriter();

writer.write("def Foo(str):")
      .write("print str");

String code = writer.toString();

There are few kinds of code writers:

AbstractCodeWriter is a lightweight template engine#

An AbstractCodeWriter can be used as a lightweight templating language. It supports interpolation, formatting, intercepting named sections of the generated content, conditionals, and loops. This removes the need to add a dependency on a Java templating engine and the need to integrate Smithy Symbols and dependency management into other templating languages. The following example uses Java 17 text blocks to generate a contiguous section of code:


// Add variables that can be referenced in templates.
writer.putContext("name", settings.getModuleName());
writer.putContext("version", settings.getModuleVersion());
writer.putContext("description", settings.getModuleDescription());

    # We ignore E203, E501 for this project due to black
    ignore = E203,E501

    name = ${name:L}
    version = ${version:L}
    description = ${description:L}
    license = Apache-2.0
    python_requires = >=3.10
    classifiers =
        Development Status :: 2 - Pre-Alpha
        Intended Audience :: Developers
        Intended Audience :: System Administrators
        Natural Language :: English
        License :: OSI Approved :: Apache Software License
        Programming Language :: Python
        Programming Language :: Python :: 3
        Programming Language :: Python :: 3 :: Only
        Programming Language :: Python :: 3.10



Various methods like write() and writeInline() take a template string and a variadic list of arguments that are interpolated, or replaced, into the expression.

In the following example, $L is interpolated and replaced with the relative argument, "there!".

CodeWriter writer = new SimpleCodeWriter();
writer.write("Hello, $L", "there!");
assert(writer.toString().equals("Hello, there!\n"));

The $ character is escaped using $$.

SimpleCodeWriter writer = new SimpleCodeWriter().write("$$L");

The default character used to start an expression is $, but this can be changed for the current state of the AbstractCodeWriter by calling setExpressionStart(char). This might be useful for programming languages that make heavy use of $ like PHP or Kotlin. A custom start character can be escaped using two start characters in a row. For example, given a custom start character of #, # can be escaped using ##.

SimpleCodeWriter writer = new SimpleCodeWriter();
writer.write("#L ##L $L", "hi");
assert(writer.toString().equals("hi #L $L\n"));


An AbstractCodeWriter supports three kinds of interpolations: relative, positional, and named. Each of these kinds of interpolations pass a value to a formatter. Formatters are named functions that accept an object as input, accepts a string that contains the current indentation (it can be ignored if not useful), and returns a string as output. AbstractCodeWriter registers two built-in formatters:

  • L (literal): Outputs a literal value of an Object using the following implementation: (1) A null value is formatted as "". (2) An empty Optional value is formatted as "". (3) A non-empty Optional value is formatted using the value inside the Optional. (3) All other values are formatted using the result of calling Java's String#valueOf.
  • S (string): Adds double quotes around the result of formatting a value first using the default literal "L" implementation described above and then wrapping the value in an escaped string safe for use in Java according to This should work for many programming languages, but this formatter can be overridden if needed.
  • C (call): Used to break up a template and execute code at specific locations. $C stands for "call" and is used to run a Runnable or Consumer<AbstractCodeWriter> that is expected to write to the same writer. Any text written to the writer is used as the interpolation result. Note that a single trailing newline is removed from the captured text. If a Runnable is provided, it is required to have a reference to the writer. A Consumer is provided a reference to the writer as a single argument. Using a Consumer makes it possible to create more generic methods for handling different sections of code.
  • …: Custom formatters can be registered using AbstractCodeWriter#putFormatter. Registering custom formatters with a writer for common formatting tasks is a great way to simplify a code generator.

Relative parameters#

Placeholders in the form of "$" followed by a formatter name are treated as relative parameters. The first instance of a relative parameter interpolates the first positional argument, the second, etc. All relative arguments must be used as part of an expression and relative interpolation cannot be mixed with positional variables.

SimpleCodeWriter writer = new SimpleCodeWriter();
writer.write("$L $L $L", "a", "b", "c");
assert(writer.toString().equals("a b c\n"));

Positional parameters#

Placeholders in the form of "$" followed by a positive number, followed by a formatter name are treated as positional parameters. The number refers to the 1-based index of the argument to interpolate. All positional arguments must be used as part of an expression and relative interpolation cannot be mixed with positional variables.

SimpleCodeWriter writer = new SimpleCodeWriter();
writer.write("$1L $2L $3L, $3L $2L $1L", "a", "b", "c");
assert(writer.toString().equals("a b c c b a\n"));

Named parameters#

Named parameters are parameters that take a value from the context of the current state. They take the following form $<variable>:<formatter>, where <variable> is a string that starts with a lowercase letter, followed by any number of [A-Za-z0-9_#$.] characters, and <formatter> is the name of a formatter.

SimpleCodeWriter writer = new SimpleCodeWriter();
writer.putContext("foo", "a");
writer.putContext("bar", "b");
writer.write("$foo:L $bar:L");
assert(writer.toString().equals("a b\n"));

Inline block alignment#

Sometimes it's necessary to maintain the exact indentation level of an interpolated property even if newlines are written when interpolating. For example, say we wanted to indent a variable list of names, Bob\nKaren\nLuis, like this:

Names: Bob

Using normal $L expansion:

writer.write("$L: $L", "Names", "Bob\nKaren\nLuis");

$L does not preserve the desired indentation, resulting in:

Names: Bob

Indentation can be preserved to match the desired list from the first example by using the inline block alignment operator (that is, putting | before the closing brace):

writer.write("$L: ${L|}", "Names", "Bob\nKaren\nLuis");

If all the characters on the line in the template leading up to the interpolation are spaces or tabs, then those characters are applied before each new line. This means that block alignment works even with tab-based languages:

    if (true) {
    """, -> w.write("Hi\nHello"))


if (true) {

Breaking up large templates with the $C formatter#

The $C formatter can be used to break up large codegen templates without losing the readability benefits of Java text blocks. The $C formatter pairs well with inline block alignment, allowing you to generate indented sections of code within a larger template.

The following example uses the call method of an AbstractCodeWriter to properly type the Function, and a method reference is provided to invoke a method that accepts the writer.

void someMethod() {
        if (true) {
        } else {

void handleTrue(CodeWriter writer) {

void handleFalse(CodeWriter writer) {


When generating code, try to show the overall structure of the code that will be generated as much as possible in larger blocks of templated text that leverage ${C|}, template conditionals (e.g., ${?foo}${/foo}), and template loops (e.g., ${#foo}${/foo}).

Pushing and popping states#

AbstractCodeWriter maintains a stack of transformation states, including the text used to indent, a prefix to add before each line, newline character, the number of times to indent, a map of context values, whether whitespace is trimmed from the end of newlines, whether the automatic insertion of newlines is disabled, the character used to start code expressions (defaults to $), and formatters.

State can be pushed onto the stack using pushState which copies the current state. Mutations can then be made to the top-most state of the AbstractCodeWriter and do not affect previous states. The previous transformation state of the AbstractCodeWriter can later be restored using popState.

SimpleCodeWriter writer = new SimpleCodeWriter();
    .setNewlinePrefix(" * ")
    .write("This is some docs.")
    .write("And more docs.\n\n\n")
    .write(" */");

The above example outputs:

 * This is some docs.
 * And more docs.
 * Foo.

AbstractCodeWriter maintains some global state that is not affected by pushState() and popState():

  • The number of successive blank lines to trim.
  • Whether a trailing newline is inserted or removed from the result of converting the AbstractCodeWriter to a string.

Limiting blank lines#

Many coding standards recommend limiting the number of successive blank lines. This can be handled automatically by AbstractCodeWriter by calling trimBlankLines(). The removal of blank lines is handled when the AbstractCodeWriter is converted to a string. Lines that consist solely of spaces or tabs are considered blank. If the number of blank lines exceeds the allowed threshold, they are omitted from the result.

SimpleCodeWriter writer = new SimpleCodeWriter();

In the above example, \n\n\n\n results in two blank lines (two newlines outputs an entirely blank line). AbstractCodeWriter trims the successive blank line, resulting in "hello\n\nhello\n" (the trailing newline is added by AbstractCodeWriter by default separately). Two blank lines could be allowed if the above example was updated to pass 2 into trimBlankLines:


Trimming trailing spaces#

Many coding standards do not allow trailing spaces on lines. Trailing spaces can be automatically trimmed from each line by calling trimTrailingSpaces().

SimpleCodeWriter writer = new SimpleCodeWriter();
writer.write("hello  ");

Code sections#

Named sections can be marked in the code writer that can be intercepted and modified by section interceptors. This gives the AbstractCodeWriter an extension system for augmenting generated code. A section of code can be captured using a block section or an inline section.

Block sections#

The primary method for creating sections of code is block sections. A block section is created by passing a string or an implementation of CodeSection to pushState(). A string gives the state a name and captures all the output written inside this state to an internal buffer. This buffer is then passed to each registered interceptor for that name. These interceptors can choose to use the default contents of the section or emit entirely different content.

SimpleCodeWriter writer = new SimpleCodeWriter();

writer.onSection("example", text -> {
    writer.write("Intercepted: " + text);

writer.write("Original contents");
assert(writer.toString().equals("Intercepted: Original contents\n"));

A better method for creating and intercepting code sections is to use an instance of a CodeSection. A CodeSection is a simple interface that is just required to return the name of the CodeSection (in fact, using a string for pushState internally creates a CodeSection).

Java records are an easy way to implement CodeSections:

record NameEvent(String sectionName, String person) implements CodeSection;

CodeInterceptors can be registered to intercept sections by class. A simple way to create one-off interceptors is using CodeSection#appender:

writer.onSection(CodeInterceptor.appender(NameEvent.class, (w, section) -> {
    w.write("$L", section.sectionName()));

writer.onSection(CodeInterceptor.appender(NameEvent.class, (w, section) -> {
    w.writeInline("Who? ");

writer.onSection(CodeInterceptor.appender(NameEvent.class, (w, section) -> {
    w.write("$L!", section.person());

When a CodeSection is given to pushState or injectState, CodeInterceptors are applied in the order they were registered.

NameEvent event = new NameEvent("Zak", "McKracken");

When applied, the writer contains the following output:

Who? McKracken!

Inline sections#

An inline section is created using a special AbstractCodeWriter interpolation format that appends "@" followed by the section name. Inline sections function just like block sections, but they can appear inline inside other content passed in calls to AbstractCodeWriter#write().

Inline sections are created in a format string inside braced arguments after the formatter. For example, ${L@foo} is an inline section that uses the literal "L" value of a relative argument as the default value of the section and allows interceptors registered for the "foo" section to make calls to the AbstractCodeWriter to modify the section.

SimpleCodeWriter writer = new SimpleCodeWriter();

// Add an intercept for the "example" section.
writer.onSection("example", text -> writer.write("Intercepted: " + text));

// Write to the writer and define an inline "example" section.
// If nothing intercepts this section, "foo" is written to it.
writer.write("Leading...${L@example}...Trailing...", "foo");

assert(writer.toString().equals("Leading...Intercepted: foo...Trailing...\n"));


An inline section that makes no calls to AbstractCodeWriter#write() expands to an empty string.

Template conditions and loops#

Conditional blocks can be defined in code writer templates using the following syntax:

    Foo is set: ${foo:L}

Assuming foo is truthy and set to "hi", then the above template outputs: "Foo is set: hi" In the above example, "?" indicates that the expression is a conditional block to check if the named context property "foo" is truthy. If it is, then the contents of the block up to the matching closing block, ${/foo}, are evaluated. If the condition is not satisfied, then contents of the block are skipped.

You can check if a named context property is falsey using "^":

    Foo is not set

Assuming foo is set to "hi", then the above template outputs nothing. If foo is falsey, then the above template outputs "Foo is not set".

Truthy and falsey values#

The following values are considered falsey:

Values that are not falsey are considered truthy.


Loops can be created to repeat a section of a template for each value stored in a list or each key value pair stored in a map. Loops are created using #.

The following template with a "foo" value of {"key1": "a", "key2": "b", "key3": "c"}:

    - ${key:L}: ${value:L} (first: ${key.first:L}, last: ${key.last:L})

Evaluates to:

- key1: a (first: true, last: false)
- key2: b (first: false, last: false)
- key3: c (first: false, last: true)

Each iteration of the loop pushes a new state in the writer that sets the following context properties:

  • key: contains the current 0-based index of an iterator or the current key of a map entry
  • value: contains the current value of an iterator or current value of a map entry
  • key.first: set to true if the loop is on the first iteration
  • key.false: set to true if the loop is on the last iteration

A custom variable name can be used in loop variable bindings. For example:

    ${#foo as key1, value1}
    - ${key1:L}: ${value1:L} (first: ${key1.first:L}, last: ${key1.last:L})

Whitespace control#

Conditional blocks and loop blocks that occur on lines that only contain whitespace are not written to the template output. For example, if foo in the following template is falsey, then the template expands to an empty string:

    Foo is set: ${foo:L}

Whitespace that comes before a template expression can be removed by putting - at the beginning of the expression.

Assuming that the first positional argument is "hi":


Expands to:


Whitespace that comes after a template expression can be removed by adding - to the end of the expression:



Expands to:


Leading whitespace cannot be removed when using inline block alignment (|). The following is invalid:

             // ^ ^ invalid combination