The Abstract Format

## 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 Fc_1 ; ... ; Name Fc_k`, where each `Fc_i` is a function clause with a pattern sequence of the same length `Arity`, then Rep(F) = `{function,LINE,Name,Arity,[Rep(Fc_1), ...,Rep(Fc_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 Ic_1 ; ... ; Ic_k end`, where each `Ic_i` is an if clause then Rep(E) = `{'if',LINE,[Rep(Ic_1), ..., Rep(Ic_k)]}`.
- If E is `case E_0 of Cc_1 ; ... ; Cc_k end`, where `E_0` is an expression and each `Cc_i` is a case clause then Rep(E) = `{'case',LINE,Rep(E_0),[Rep(Cc_1), ..., Rep(Cc_k)]}`.
- If E is `try B catch Tc_1 ; ... ; Tc_k end`, where `B` is a body and each `Tc_i` is a catch clause then Rep(E) = `{'try',LINE,Rep(B),[],[Rep(Tc_1), ..., Rep(Tc_k)],[]}`.
- If E is `try B of Cc_1 ; ... ; Cc_k catch Tc_1 ; ... ; Tc_n end`, where `B` is a body, each `Cc_i` is a case clause and each `Tc_j` is a catch clause then Rep(E) = `{'try',LINE,Rep(B),[Rep(Cc_1), ..., Rep(Cc_k)],[Rep(Tc_1), ..., Rep(Tc_n)],[]}`.
- If E is `try B after A end`, where `B` and `A` are bodies then Rep(E) = `{'try',LINE,Rep(B),[],[],Rep(A)}`.
- If E is `try B of Cc_1 ; ... ; Cc_k after A end`, where `B` and `A` are a bodies and each `Cc_i` is a case clause then Rep(E) = `{'try',LINE,Rep(B),[Rep(Cc_1), ..., Rep(Cc_k)],[],Rep(A)}`.
- If E is `try B catch Tc_1 ; ... ; Tc_k after A end`, where `B` and `A` are bodies and each `Tc_i` is a catch clause then Rep(E) = `{'try',LINE,Rep(B),[],[Rep(Tc_1), ..., Rep(Tc_k)],Rep(A)}`.
- If E is `try B of Cc_1 ; ... ; Cc_k catch Tc_1 ; ... ; Tc_n after A end`, where `B` and `A` are a bodies, each `Cc_i` is a case clause and each `Tc_j` is a catch clause then Rep(E) = `{'try',LINE,Rep(B),[Rep(Cc_1), ..., Rep(Cc_k)],[Rep(Tc_1), ..., Rep(Tc_n)],Rep(A)}`.
- If E is `receive Cc_1 ; ... ; Cc_k end`, where each `Cc_i` is a case clause then Rep(E) = `{'receive',LINE,[Rep(Cc_1), ..., Rep(Cc_k)]}`.
- If E is `receive Cc_1 ; ... ; Cc_k after E_0 -> B_t end`, where each `Cc_i` is a case clause, `E_0` is an expression and `B_t` is a body, then Rep(E) = `{'receive',LINE,[Rep(Cc_1), ..., Rep(Cc_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 Module:Name/Arity`, then Rep(E) = `{'fun',LINE,{function,Module,Name,Arity}}`.
- If E is `fun Fc_1 ; ... ; Fc_k end` where each `Fc_i` is a function clause then Rep(E) = `{'fun',LINE,{clauses,[Rep(Fc_1), ..., Rep(Fc_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 Clauses

There are function clauses, if clauses, case clauses and catch clauses.

A clause `C` is one of the following alternatives:

- If C is a function clause `( Ps ) -> B` where `Ps` is a pattern sequence and `B` is a body, then Rep(C) = `{clause,LINE,Rep(Ps),[],Rep(B)}`.
- If C is a function clause `( Ps ) when Gs -> B` where `Ps` is a pattern sequence, `Gs` is a guard sequence and `B` is a body, then Rep(C) = `{clause,LINE,Rep(Ps),Rep(Gs),Rep(B)}`.
- If C is an if clause `Gs -> B` where `Gs` is a guard sequence and `B` is a body, then Rep(C) = `{clause,LINE,[],Rep(Gs),Rep(B)}`.
- If C is a case clause `P -> B` where `P` is a pattern and `B` is a body, then Rep(C) = `{clause,LINE,[Rep(P)],[],Rep(B)}`.
- If C is a case clause `P when Gs -> B` where `P` is a pattern, `Gs` is a guard sequence and `B` is a body, then Rep(C) = `{clause,LINE,[Rep(P)],Rep(Gs),Rep(B)}`.
- If C is a catch clause `P -> B` where `P` is a pattern and `B` is a body, then Rep(C) = `{clause,LINE,[Rep({throw,P,_})],[],Rep(B)}`.
- If C is a catch clause `X : P -> B` where `X` is an atomic literal or a variable pattern, `P` is a pattern and `B` is a body, then Rep(C) = `{clause,LINE,[Rep({X,P,_})],[],Rep(B)}`.
- If C is a catch clause `P when Gs -> B` where `P` is a pattern, `Gs` is a guard sequence and `B` is a body, then Rep(C) = `{clause,LINE,[Rep({throw,P,_})],Rep(Gs),Rep(B)}`.
- If C is a catch clause `X : P when Gs -> B` where `X` is an atomic literal or a variable pattern, `P` is a pattern, `Gs` is a guard sequence and `B` is a body, then Rep(C) = `{clause,LINE,[Rep({X,P,_})],Rep(Gs),Rep(B)}`.

### 4.6 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.

### 4.7 The abstract format after preprocessing

The compilation option `debug_info` can be given to the compiler to have the abstract code stored in the `abstract_code` chunk in the BEAM file (for debugging purposes).

In OTP R9C and later, the `abstract_code` chunk will contain

`{raw_abstract_v1,AbstractCode}`

where `AbstractCode` is the abstract code as described in this document.

In releases of OTP prior to R9C, the abstract code after some more processing was stored in the BEAM file. The first element of the tuple would be either `abstract_v1` (R7B) or `abstract_v2` (R8B).

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