## 4 The Abstract Format

This document describes the standard representation of parse trees for Erlang programs as Erlang terms. This representation is known as the abstract format. Functions dealing with such parse trees are `compile:forms/[1,2]` and functions in the modules `epp`, `erl_eval`, `erl_lint`, `erl_pp`, `erl_parse`, and `io`. They are also used as input and output for parse transforms (see the module `compile`).

We use the function `Rep` to denote the mapping from an Erlang source construct `C` to its abstract format representation `R`, and write `R = Rep(C)`.

The word `LINE` below represents an integer, and denotes the number of the line in the source file where the construction occurred. Several instances of `LINE` in the same construction may denote different lines.

Since operators are not terms in their own right, when operators are mentioned below, the representation of an operator should be taken to be the atom with a printname consisting of the same characters as the operator.

### 4.1 Module declarations and forms

A module declaration consists of a sequence of forms that are either function declarations or attributes.

- If D is a module declaration consisting of the forms `F_1`, ..., `F_k`, then Rep(D) = `[Rep(F_1), ..., Rep(F_k)]`.
- If F is an attribute `-module(Mod)`, then Rep(F) = `{attribute,LINE,module,Mod}`.
- If F is an attribute `-export([Fun_1/A_1, ..., Fun_k/A_k])`, then Rep(F) = `{attribute,LINE,export,[{Fun_1,A_1}, ..., {Fun_k,A_k}]}`.
- If F is an attribute `-import(Mod,[Fun_1/A_1, ..., Fun_k/A_k])`, then Rep(F) = `{attribute,LINE,import,{Mod,[{Fun_1,A_1}, ..., {Fun_k,A_k}]}}`.
- If F is an attribute `-compile(Options)`, then Rep(F) = `{attribute,LINE,compile,Options}`.
- If F is an attribute `-file(File,Line)`, then Rep(F) = `{attribute,LINE,file,{File,Line}}`.
- If F is a record declaration `-record(Name,{V_1, ..., V_k})`, then Rep(F) = `{attribute,LINE,record,{Name,[Rep(V_1), ..., Rep(V_k)]}}`. For Rep(V), see below.
- If F is a wild attribute `-A(T)`, then Rep(F) = `{attribute,LINE,A,T}`.
- If F is a function declaration `Name(Ps_1) when Gs_1 -> B_1 ; ... ; Name(Ps_k) when Gs_k -> B_k end`, where each `Ps_i`, `Gs_i` and `B_i` is a pattern sequence, a guard sequence and a body, respectively, and each `Ps_i` has the same length `Arity`, then Rep(F) = `{function,LINE,Name,Arity, [{clause,LINE,Rep(Ps_1),Rep(Gs_1),Rep(B_1)}, ..., {clause,LINE,Rep(Ps_k),Rep(Gs_k),Rep(B_k)}]}`.

#### 4.1.1 Record fields

Each field in a record declaration may have an optional explicit default initializer expression

- If V is `A`, then Rep(V) = `{record_field,LINE,Rep(A)}`.
- If V is `A = E`, then Rep(V) = `{record_field,LINE,Rep(A),Rep(E)}`.

#### 4.1.2 Representation of parse errors and end of file

In addition to the representations of forms, the list that represents a module declaration (as returned by functions in `erl_parse` and `epp`) may contain tuples `{error,E}`, denoting syntactically incorrect forms, and `{eof,LINE}`, denoting an end of stream encountered before a complete form had been parsed.

### 4.2 Atomic literals

There are five kinds of atomic literals, which are represented in the same way in patterns, expressions and guard expressions:

- If L is an integer or character literal, then Rep(L) = `{integer,LINE,L}`.
- If L is a float literal, then Rep(L) = `{float,LINE,L}`.
- If L is a string literal consisting of the characters `C_1`, ..., `C_k`, then Rep(L) = `{string,LINE,[C_1, ..., C_k]}`.
- If L is an atom literal, then Rep(L) = `{atom,LINE,L}`.

Note that negative integer and float literals do not occur as such; they are parsed as an application of the unary negation operator.

### 4.3 Patterns

If `Ps` is a sequence of patterns `P_1, ..., P_k`, then Rep(Ps) = `[Rep(P_1), ..., Rep(P_k)]`. Such sequences occur as the list of arguments to a function or fun.

Individual patterns are represented as follows:

- If P is an atomic literal L, then Rep(P) = Rep(L),
- If P is a compound pattern `P_1 = P_2`, then Rep(P) = `{match,LINE,Rep(P_1),Rep(P_2)}`.
- If P is a variable pattern `V`, then Rep(P) = `{var,LINE,A}`, where A is an atom with a printname consisting of the same characters as `V`.
- If P is a universal pattern `_`, then Rep(P) = `{var,LINE,'_'}`.
- If P is a tuple pattern `{P_1, ..., P_k}`, then Rep(P) = `{tuple,LINE,[Rep(P_1), ..., Rep(P_k)]}`.
- If P is a nil pattern `[]`, then Rep(P) = `{nil,LINE}`.
- If P is a cons pattern `[P_h | P_t]`, then Rep(P) = `{cons,LINE,Rep(P_h),Rep(P_t)}`.
- If E is a binary pattern `<<P_1:Size_1/TSL_1, ..., P_k:Size_k/TSL_k>>`, then Rep(E) = `{bin,LINE,[{bin_element,LINE,Rep(P_1),Rep(Size_1),Rep(TSL_1)}, ..., {bin_element,LINE,Rep(P_k),Rep(Size_k),Rep(TSL_k)}]}`. For Rep(TSL), see below. An omitted `Size` is represented by `default`. An omitted `TSL` (type specifier list) is represented by `default`.
- If P is `P_1 Op P_2`, where `Op` is a binary operator (this is either an occurrence of `++` applied to a literal string or character list, or an occurrence of an expression that can be evaluated to a number at compile time), then Rep(P) = `{op,LINE,Op,Rep(P_1),Rep(P_2)}`.
- If P is `Op P_0`, where `Op` is a unary operator (this is an occurrence of an expression that can be evaluated to a number at compile time), then Rep(P) = `{op,LINE,Op,Rep(P_0)}`.
- If P is a record pattern `#Name{Field_1=P_1, ..., Field_k=P_k}`, then Rep(P) = `{record,LINE,Name, [{record_field,LINE,Rep(Field_1),Rep(P_1)}, ..., {record_field,LINE,Rep(Field_k),Rep(P_k)}]}`.

Note that every pattern has the same source form as some expression, and is represented the same way as the corresponding expression.

### 4.4 Expressions

A body B is a sequence of expressions `E_1, ..., E_k`, and Rep(B) = `[Rep(E_1), ..., Rep(E_k)]`.

An expression E is one of the following alternatives:

- If P is an atomic literal `L`, then Rep(P) = Rep(L).
- If E is `P = E_0`, then Rep(E) = `{match,LINE,Rep(P),Rep(E_0)}`.
- If E is a variable `V`, then Rep(E) = `{var,LINE,A}`, where `A` is an atom with a printname consisting of the same characters as `V`.
- If E is a tuple skeleton `{E_1, ..., E_k}`, then Rep(E) = `{tuple,LINE,[Rep(E_1), ..., Rep(E_k)]}`.
- If E is `[]`, then Rep(E) = `{nil,LINE}`.
- If E is a cons skeleton `[E_h | E_t]`, then Rep(E) = `{cons,LINE,Rep(E_h),Rep(E_t)}`.
- If E is a binary constructor `<<V_1:Size_1/TSL_1, ..., V_k:Size_k/TSL_k>>`, then Rep(E) = `{bin,LINE,[{bin_element,LINE,Rep(V_1),Rep(Size_1),Rep(TSL_1)}, ..., {bin_element,LINE,Rep(V_k),Rep(Size_k),Rep(TSL_k)}]}`. For Rep(TSL), see below. An omitted `Size` is represented by `default`. An omitted `TSL` (type specifier list) is represented by `default`.
- If E is `E_1 Op E_2`, where `Op` is a binary operator, then Rep(E) = `{op,LINE,Op,Rep(E_1),Rep(E_2)}`.
- If E is `Op E_0`, where `Op` is a unary operator, then Rep(E) = `{op,LINE,Op,Rep(E_0)}`.
- If E is `#Name{Field_1=E_1, ..., Field_k=E_k}`, then Rep(E) = `{record,LINE,Name, [{record_field,LINE,Rep(Field_1),Rep(E_1)}, ..., {record_field,LINE,Rep(Field_k),Rep(E_k)}]}`.
- If E is `E_0#Name{Field_1=E_1, ..., Field_k=E_k}`, then Rep(E) = `{record,LINE,Rep(E_0),Name, [{record_field,LINE,Rep(Field_1),Rep(E_1)}, ..., {record_field,LINE,Rep(Field_k),Rep(E_k)}]}`.
- If E is `#Name.Field`, then Rep(E) = `{record_index,LINE,Name,Rep(Field)}`.
- If E is `E_0#Name.Field`, then Rep(E) = `{record_field,LINE,Rep(E_0),Name,Rep(Field)}`.
- If E is `catch E_0`, then Rep(E) = `{'catch',LINE,Rep(E_0)}`.
- If E is `E_0(E_1, ..., E_k)`, then Rep(E) = `{call,LINE,Rep(E_0),[Rep(E_1), ..., Rep(E_k)]}`.
- If E is `E_m:E_0(E_1, ..., E_k)`, then Rep(E) = `{call,LINE,{remote,LINE,Rep(E_m),Rep(E_0)},[Rep(E_1), ..., Rep(E_k)]}`.
- If E is a list comprehension `[E_0 || W_1, ..., W_k]`, where each `W_i` is a generator or a filter, then Rep(E) = `{lc,LINE,Rep(E_0),[Rep(W_1), ..., Rep(W_k)]}`. For Rep(W), see below.
- If E is `begin B end`, where `B` is a body, then Rep(E) = `{block,LINE,Rep(B)}`.
- If E is `if Gs_1 -> B_1 ; ... ; Gs_k -> B_k end`, where each `Gs_i` and `B_i` is a guard sequence and a body, respectively, then Rep(E) = `{'if',LINE,[{clause,LINE,[],Rep(Gs_1),Rep(B_1)}, ..., {clause,LINE,[],Rep(Gs_k),Rep(B_k)}]}`.
- If E is `case E_0 of P_1 when Gs_1 -> B_1 ; ... ; P_k when Gs_k -> B_k end`, where `E_0` is an expression and each `P_i`, `Gs_i` and `B_i` is a pattern, a guard sequence and a body, respectively, then Rep(E) = `{'case',LINE,Rep(E_0), [{clause,LINE,[Rep(P_1)],Rep(Gs_1),Rep(B_1)}, ..., {clause,LINE,[Rep(P_k)],Rep(Gs_k),Rep(B_k)}]}`.
- If E is `receive P_1 when Gs_1 -> B_1 ; ... ; P_k when Gs_k -> B_k end`, where each `P_i`, `Gs_i` and `B_i` is a pattern, a guard sequence and a body, respectively, then Rep(E) = `{'receive',LINE, [{clause,LINE,[Rep(P_1)],Rep(Gs_1),Rep(B_1)}, ..., {clause,LINE,[Rep(P_k)],Rep(Gs_k),Rep(B_k)}]}`.
- If E is `receive P_1 when Gs_1 -> B_1 ; ... ; P_k when Gs_k -> B_k after E_0 -> B_t end`, where each `P_i`, `Gs_i` and `B_i` is a pattern, a guard sequence and a body, respectively, `E_0` is an expression and `B_t` is a body, then Rep(E) = `{'receive',LINE, [{clause,LINE,[Rep(P_1)],Rep(Gs_1),Rep(B_1)}, ..., {clause,LINE,[Rep(P_k)],Rep(Gs_k),Rep(B_k)}], Rep(E_0),Rep(B_t)}`.
- If E is `fun Name/Arity`, then Rep(E) = `{'fun',LINE,{function,Name,Arity}}`.
- If E is `fun Ps_1 when Gs_1 -> B_1 ; ... ; Ps_k when Gs_k -> B_k end`, where each `Ps_i`, `Gs_i` and `B_i` is a pattern sequence, a guard sequence and a body, respectively, then Rep(E) = `{'fun',LINE,{clauses, [{clause,LINE,[Rep(Ps_1)],Rep(Gs_1),Rep(B_1)}, ..., {clause,LINE,[Rep(Ps_k)],Rep(Gs_k),Rep(B_k)}]}}`.
- If E is `query [E_0 || W_1, ..., W_k] end`, where each `W_i` is a generator or a filter, then Rep(E) = `{'query',LINE,{lc,LINE,Rep(E_0),[Rep(W_1), ..., Rep(W_k)]}}`. For Rep(W), see below.
- If E is `E_0.Field`, a Mnesia record access inside a query, then Rep(E) = `{record_field,LINE,Rep(E_0),Rep(Field)}`.
- If E is `( E_0 )`, then Rep(E) = `Rep(E_0)`, i.e., parenthesized expressions cannot be distinguished from their bodies.

#### 4.4.1 Generators and filters

When W is a generator or a filter (in the body of a list comprehension), then:

- If W is a generator `P <- E`, where `P` is a pattern and `E` is an expression, then Rep(W) = `{generate,LINE,Rep(P),Rep(E)}`.
- If W is a filter `E`, which is an expression, then Rep(W) = `Rep(E)`.

#### 4.4.2 Binary element type specifiers

A type specifier list TSL for a binary element is a sequence of type specifiers `TS_1 - ... - TS_k`. Rep(TSL) = `[Rep(TS_1), ..., Rep(TS_k)]`.

When TS is a type specifier for a binary element, then:

- If TS is an atom `A`, Rep(TS) = `A`.
- If TS is a couple `A:Value` where `A` is an atom and `Value` is an integer, Rep(TS) = `{A, Value}`.

### 4.5 Guards

A guard Gs is a nonempty sequence of guard tests `G_1, ..., G_k`, and Rep(Gs) = `[Rep(G_1), ..., Rep(G_k)]`.

A guard sequence Gss is a sequence of guards `Gs_1; ...; Gs_k`, and Rep(Gss) = `[Rep(Gs_1), ..., Rep(Gs_k)]`. If the guard sequence is empty, Rep(Gss) = `[]`.

A guard test G is either `true`, an application of a BIF to a sequence of guard expressions (syntactically this includes guard record tests), or a binary operator applied to two guard expressions.

- If G is `true`, then Rep(G) = `{atom,LINE,true}`.
- If G is an application `A(E_1, ..., E_k)`, where `A` is an atom and `E_1`, ..., `E_k` are guard expressions, then Rep(G) = `{call,LINE,{atom,LINE,A},[Rep(E_1), ..., Rep(E_k)]}`.
- If G is an operator expression `E_1 Op E_2`, where `Op` is a binary operator, and `E_1`, `E_2` are guard expressions, then Rep(G) = `{op,LINE,Op,Rep(E_1),Rep(E_2)}`.

All guard expressions are expressions and are represented in the same way as the corresponding expressions.

### 4.6 The abstract format after preprocessing

When Erlang source code is compiled, the abstract code, after some preprocessing, is stored as the `abstract_code` chunk in the BEAM file, for debugging purposes. The version of the preprocessed format in OTP R7 is called `abstract_v1`, in R8 `abstract_v2`. The preprocessing changes the representation so it becomes slightly incompatible with the format described above. The differences are:

- BIF calls in guards are translated to the `{remote, ...}` form (which is not allowed in source form).
- Explicit funs are translated to a tuple with an extra element (new in R7): `{'fun',LINE,{clauses, Clauses},Extra}`. The form of this extra element may change from one OTP release to the next.
- Implicit funs are translated to a tuple with an extra element (new in R8): `{'fun',LINE,{function,Name,Arity},Extra}`.

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