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# cerl_trees (compiler v9.0)

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Basic functions on Core Erlang abstract syntax trees.

#### Note

The public interface of the Erlang compiler can be found in module `compile`.

This module is an internal part of the compiler. Its API is not guaranteed to remain compatible between releases.

Syntax trees are defined in the module `cerl`.

# Summary

## Types

cerl()

## Functions

depth(Tree)

Returns the length of the longest path in the tree.

fold(Function, Unit, Term)

Does a fold operation over the nodes of the tree.

free_variables(Tree)

Like `variables/1`, but only includes variables that are free in the tree.

get_label(Tree)

Retrieves the label for `Tree`.

label(T)

Equivalent to label(Tree, 0).

label(Tree, N)

Labels each expression in the tree.

map(Function, Tree)

Maps a function onto the nodes of a tree.

mapfold(Function, Initial, Tree)

Does a combined map/fold operation on the nodes of the tree.

mapfold(Pre, Post, Initial, Tree)

Does a combined map/fold operation on the nodes of the tree.

next_free_variable_name(Tree)

Returns a integer variable name higher than any other integer variable name in the syntax tree.

size(Tree)

Returns the number of nodes in `Tree`.

variables(Tree)

Returns an ordered-set list of the names of all variables in the syntax tree (including function-name variables.)

# Types

# cerl()

(not exported)

    -type cerl() :: cerl:cerl().

# Functions

# depth(Tree)

    -spec depth(Tree :: cerl()) -> non_neg_integer().

Returns the length of the longest path in the tree.

A leaf node has depth zero, the tree representing "`{foo, bar}`" has depth one, and so on.

# fold(Function, Unit, Term)

    -spec fold(Function :: fun((cerl(), term()) -> term()), Unit :: term(), Term :: cerl()) -> term().

Does a fold operation over the nodes of the tree.

The result is the value of `Function(X1, Function(X2, ... Function(Xn, Unit) ... ))`, where `X1, ..., Xn` are the nodes of `Tree` in a post-order traversal.

See also: `mapfold/3`.

# free_variables(Tree)

    -spec free_variables(Tree :: cerl()) -> [cerl:var_name()].

Like `variables/1`, but only includes variables that are free in the tree.

See also: `next_free_variable_name/1`, `variables/1`.

# get_label(Tree)

    -spec get_label(Tree :: cerl()) -> top | integer().

Retrieves the label for `Tree`.

An exception is thrown if `Tree` does not have a label, or if `Tree` does not represent a well-formed Core Erlang syntax tree.

# label(T)

    -spec label(cerl()) -> {cerl(), integer()}.

Equivalent to label(Tree, 0).

# label(Tree, N)

    -spec label(Tree :: cerl(), N :: integer()) -> {cerl(), integer()}.

Labels each expression in the tree.

A term `{label, L}` is prefixed to the annotation list of each expression node, where L is a unique number for every node, except for variables (and function name variables) which get the same label if they represent the same variable. Constant literal nodes are not labeled.

The returned value is a tuple `{NewTree, Max}`, where `NewTree` is the labeled tree and `Max` is 1 plus the largest label value used. All previous annotation terms of the form `{label, X}` are deleted.

The values of L used in the tree is a dense range from `N` to `Max - 1`, where `N =< Max =< N + size(Tree)`. Note that it is possible that no labels are used at all, i.e., `N = Max`.

Note: All instances of free variables will be given distinct labels.

See also: `label/1`, `size/1`.

# map(Function, Tree)

    -spec map(Function :: fun((cerl()) -> cerl()), Tree :: cerl()) -> cerl().

Maps a function onto the nodes of a tree.

This replaces each node in the tree by the result of applying the given function on the original node, bottom-up.

See also: `mapfold/3`.

# mapfold(Function, Initial, Tree)

    -spec mapfold(Function :: fun((cerl(), term()) -> {cerl(), term()}), Initial :: term(), Tree :: cerl()) ->
                     {cerl(), term()}.

Does a combined map/fold operation on the nodes of the tree.

This is similar to `map/2`, but also propagates a value from each application of `Function` to the next, starting with the given value `Initial`, while doing a post-order traversal of the tree, much like `fold/3`.

This is equivalent to `mapfold/4` with an identity function as the pre-operation.

See also: `fold/3`, `map/2`, `mapfold/4`.

# mapfold(Pre, Post, Initial, Tree)

    -spec mapfold(Pre :: fun((cerl(), term()) -> {cerl(), term()} | skip),
                  Post :: fun((cerl(), term()) -> {cerl(), term()}),
                  Initial :: term(),
                  Tree :: cerl()) ->
                     {cerl(), term()}.

Does a combined map/fold operation on the nodes of the tree.

It begins by calling `Pre` on the tree, using the `Initial` value. `Pre` must either return a tree with an updated accumulator or the atom `skip`.

If a tree is returned, this function deconstructs the top node of the returned tree and recurses on the children, using the returned value as the new initial and carrying the returned values from one call to the next. Finally it reassembles the top node from the children, calls `Post` on it and returns the result.

If `skip` is returned, it returns the tree and accumulator as is.

# next_free_variable_name(Tree)

    -spec next_free_variable_name(Tree :: cerl()) -> integer().

Returns a integer variable name higher than any other integer variable name in the syntax tree.

An exception is thrown if `Tree` does not represent a well-formed Core Erlang syntax tree.

See also: `free_variables/1`, `variables/1`.

# size(Tree)

    -spec size(Tree :: cerl()) -> non_neg_integer().

Returns the number of nodes in `Tree`.

# variables(Tree)

    -spec variables(Tree :: cerl()) -> [cerl:var_name()].

Returns an ordered-set list of the names of all variables in the syntax tree (including function-name variables.)

An exception is thrown if `Tree` does not represent a well-formed Core Erlang syntax tree.

See also: `free_variables/1`, `next_free_variable_name/1`.