# 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)

### AUTHORS

Robert Virding - support@erlang.ericsson.se

stdlib 1.14.3
Copyright © 1991-2007 Ericsson AB