Prints cross reference information between modules.

The xref task expects a mode as first argument:

$ mix xref MODE

All available modes are discussed below.

This task is automatically reenabled, so you can print information multiple times in the same Mix invocation.

mix xref callers MODULE

Prints all callers of the given MODULE. Example:

$ mix xref callers MyMod

mix xref trace FILE

Compiles the given file listing all dependencies within the same app. It includes the type and line for each one. Example:

$ mix xref trace lib/my_app/router.ex

The --label option may be given to keep only certain traces (compile, runtime or export):

$ mix xref trace lib/my_app/router.ex --label compile

If you have an umbrella application, we also recommend using the --include-siblings flag to see the dependencies on other umbrella applications.

Example

Imagine the given file lib/b.ex:

defmodule B do
  import A
  A.macro()
  macro()
  A.fun()
  fun()
  def calls_macro, do: A.macro()
  def calls_fun, do: A.fun()
  def calls_struct, do: %A{}
end

mix xref trace will print:

lib/b.ex:2: require A (export)
lib/b.ex:3: call A.macro/0 (compile)
lib/b.ex:4: import A.macro/0 (compile)
lib/b.ex:5: call A.fun/0 (compile)
lib/b.ex:6: call A.fun/0 (compile)
lib/b.ex:6: import A.fun/0 (compile)
lib/b.ex:7: call A.macro/0 (compile)
lib/b.ex:8: call A.fun/0 (runtime)
lib/b.ex:9: struct A (export)

mix xref graph

Prints a file dependency graph where an edge from A to B indicates that A (source) depends on B (sink).

$ mix xref graph --format stats

The following options are accepted:

• --exclude - paths to exclude

• --label - only shows relationships with the given label. The labels are "compile", "export" and "runtime". By default, the --label option simply filters the printed graph to show only relationships with the given label. If you want to effectively filter the graph, you can pass the --only-direct flag. There is also a special label called "compile-connected" that keeps only compile-time files with at least one transitive dependency. See "Dependencies types" section below.

• --only-direct - keeps only files with the direct relationship given by --label

• --only-nodes - only shows the node names (no edges). Generally useful with the --sink flag

• --source - displays all files that the given source file references (directly or indirectly)

• --sink - displays all files that reference the given file (directly or indirectly)

• --min-cycle-size - controls the minimum cycle size on formats like stats and cycles

• --format - can be set to one of:

  • pretty - prints the graph to the terminal using Unicode characters. Each prints each file followed by the files it depends on. This is the default except on Windows;

  • plain - the same as pretty except ASCII characters are used instead of Unicode characters. This is the default on Windows;

  • stats - prints general statistics about the graph;

  • cycles - prints all cycles in the graph;

  • dot - produces a DOT graph description in xref_graph.dot in the current directory. Warning: this will override any previously generated file

The --source and --sink options are particularly useful when trying to understand how the modules in a particular file interact with the whole system. You can combine those options with --label and --only-nodes to get all files that exhibit a certain property, for example:

# To show all compile-time relationships
mix xref graph --label compile

# To get the tree that depend on lib/foo.ex at compile time
mix xref graph --label compile --sink lib/foo.ex

# To get all files that depend on lib/foo.ex at compile time
mix xref graph --label compile --sink lib/foo.ex --only-nodes

# To get all paths between two files
mix xref graph --source lib/foo.ex --sink lib/bar.ex

# To show general statistics about the graph
mix xref graph --format stats

Understanding the printed graph

When mix xref graph runs, it will print a tree of the following format. Imagine the following code:

# lib/a.ex
defmodule A do
  IO.puts B.hello()
end

# lib/b.ex
defmodule B do
  def hello, do: C.world()
end

# lib/c.ex
defmodule C do
  def world, do: "hello world"
end

It will print:

$ mix xref graph
lib/a.ex
└── lib/b.ex (compile)
lib/b.ex
└── lib/c.ex
lib/c.ex

This tree means that lib/a.ex depends on lib/b.ex at compile time. And lib/b.ex depends on lib/c.ex at runtime. This is often problematic because if lib/c.ex changes, lib/a.ex also has to recompile due to this indirect compile time dependency. When you pass --label compile, the graph shows only the compile-time dependencies:

$ mix xref graph --label compile
lib/a.ex
└── lib/b.ex (compile)

The --label compile flag removes all non-compile dependencies. However, this can be misleading because having direct compile time dependencies is not necessarily an issue. The biggest concern, as mentioned above, are the transitive compile time dependencies. You can get all compile time dependencies that cause transitive compile time dependencies by using --label compile-connected:

$ mix xref graph --label compile-connected
lib/a.ex
└── lib/b.ex (compile)

The above says lib/a.ex depends on lib/b.ex and that causes transitive compile time dependencies - as we know, lib/a.ex also depends on lib/c.ex. We can retrieve those transitive dependencies by passing lib/b.ex as --source to mix xref graph:

$ mix xref graph --source lib/b.ex
lib/b.ex
└── lib/c.ex

Similarly, you can use the --label compile and the --sink flag to find all compile time dependencies that will recompile once the sink changes:

$ mix xref graph --label compile --sink lib/c.ex
lib/a.ex
└── lib/b.ex (compile)

Dependencies types

Elixir tracks three types of dependencies between modules: compile, exports, and runtime. If a module has a compile time dependency on another module, the caller module has to be recompiled whenever the callee changes. Compile-time dependencies are typically added when using macros or when invoking functions in the module body (outside of functions). You can list all dependencies in a file by running mix xref trace path/to/file.ex.

Exports dependencies are compile time dependencies on the module API, namely structs and its public definitions. For example, if you import a module but only use its functions, it is an export dependency. If you use a struct, it is an export dependency too. Export dependencies are only recompiled if the module API changes. Note, however, that compile time dependencies have higher precedence than exports. Therefore if you import a module and use its macros, it is a compile time dependency.

Runtime dependencies are added whenever you invoke another module inside a function. Modules with runtime dependencies do not have to be compiled when the callee changes, unless there is a transitive compile or an outdated export time dependency between them. The option --label compile-connected can be used to find the first case.

Shared options

Those options are shared across all modes:

• --fail-above - generates a failure if the relevant metric is above the given threshold. This metric is the number of references, except for --format cycles where it is the number of cycles, and --format stats which has none.

• --include-siblings - includes dependencies that have :in_umbrella set to true in the current project in the reports. This can be used to find callers or to analyze graphs between projects

• --no-compile - does not compile even if files require compilation

• --no-deps-check - does not check dependencies

• --no-archives-check - does not check archives

• --no-elixir-version-check - does not check the Elixir version from mix.exs