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# erl_pp

### MODULE

erl_pp

### MODULE SUMMARY

The Erlang Pretty Printer

### DESCRIPTION

The functions in this module are used to generate aesthetically attractive representations of abstract forms, which are suitable for printing. All functions return (possibly deep) lists of characters and generate an error if the form is wrong.

All functions can have an optional argument which specifies a hook that is called if an attempt is made to print an unknown form.

### EXPORTS

form(Form) -> DeepCharList
form(Form, HookFunction) -> DeepCharList

Types:

Form = term()
HookFunction = see separate description below.
DeepCharList = [char()|DeepCharList]

Pretty prints a Form which is an abstract form of a type which is returned by erl_parse:parse_form.

attribute(Attribute) -> DeepCharList
attribute(Attribute, HookFunction) -> DeepCharList

Types:

Attribute = term()
HookFunction = see separate description below.
DeepCharList = [char()|DeepCharList]

The same as form, but only for the attribute Attribute.

function(Function) -> DeepCharList
function(Function, HookFunction) -> DeepCharList

Types:

Function = term()
HookFunction = see separate description below.
DeepCharList = [char()|DeepCharList]

The same as form, but only for the function Function.

guard(Guard) -> DeepCharList
guard(Guard, HookFunction) -> DeepCharList

Types:

Form = term()
HookFunction = see separate description below.
DeepCharList = [char()|DeepCharList]

The same as form, but only for the guard test Guard.

exprs(Expressions) -> DeepCharList
exprs(Expressions, HookFunction) -> DeepCharList
exprs(Expressions, Indent, HookFunction) -> DeepCharList

Types:

Expressions = term()
HookFunction = see separate description below.
Indent = integer()
DeepCharList = [char()|DeepCharList]

The same as form, but only for the sequence of expressions in Expressions.

expr(Expression) -> DeepCharList
expr(Expression, HookFunction) -> DeepCharList
expr(Expression, Indent, HookFunction) -> DeepCharList
expr(Expression, Indent, Precedence, HookFunction) ->-> DeepCharList

Types:

Expression = term()
HookFunction = see separate description below.
Indent = integer()
Precedence =
DeepCharList = [char()|DeepCharList]

This function prints one expression. It is useful for implementing hooks (see below).

### Unknown Expression Hooks

The optional argument HookFunction, shown in the functions described above, defines a function which is called when an unknown form occurs where there should be a valid expression. It can have the following formats:

Function

  The hook function is called by:

      Function(Expr,
               CurrentIndentation,
               CurrentPrecedence,
               HookFunction)

none

  There is no hook function

The called hook function should return a (possibly deep) list of characters. expr/4 is useful in a hook.

If CurrentIndentation is negative, there will be no line breaks and only a space is used as a separator.

### Bugs

It should be possible to have hook functions for unknown forms at places other than expressions.

### See Also

io(3), erl_parse(3), erl_eval(3)