# Julia ASTs¶

Julia has two AST representations. First there is a surface syntax AST returned by the parser (e.g. the parse() function), and manipulated by macros. It is a structured representation of code as it is written, constructed by julia-parser.scm from a character stream. Next there is a lowered AST which is used by type inference and code generation. In the lowered form, there are generally fewer types of nodes, all macros are expanded, and all control flow is converted to explicit branches and sequences of statements. The lowered form is constructed by julia-syntax.scm.

First we will focus on the lowered form, since it is more important to the compiler. It is also less obvious to the human, since it results from a significant rearrangement of the input syntax.

## Lowered form¶

The following data types exist in lowered ASTs:

Expr
has a node type indicated by the head field, and an args field which is a Vector{Any} of subexpressions.
Symbol
used to name local variables and static parameters within a function.
LambdaStaticData
wraps the AST of each function, including inner functions.
LineNumberNode
LabelNode
branch target, a consecutively-numbered integer starting at 0
GotoNode
unconditional branch
QuoteNode
wraps an arbitrary value to reference as data. For example, the function f() = :a contains a QuoteNode whose value field is the symbol a, in order to return the symbol itself instead of evaluating it.
GlobalRef
refers to global variable name in module mod
TopNode
forces a name to be resolved as a global in Base. This is now mostly redundant with GlobalRef(Base, :x).
SymbolNode
used to annotate a local variable with a type
GenSym
refers to a consecutively-numbered (starting at 0) static single assignment (SSA) variable inserted by the compiler.
NewvarNode
marks a point where a closed variable needs to have a new box allocated.

### Expr types¶

These symbols appear in the head field of Exprs in lowered form.

call
function call. args[1] is the function to call, args[2:end] are the arguments.
line
line number and file name metadata. Unlike a LineNumberNode, can also contain a file name.
gotoifnot
conditional branch. If args[1] is false, goes to label identified in args[2].
=
assignment
method

adds a method to a generic function and assigns the result if necessary.

args[1] - function name (symbol), or a GlobalRef, or an Expr with head kw. In the (kw f) case, the method is actually a keyword argument sorting function for f. It will be stored instead in generic_function->env->kwsorter.

If method has only one argument, it corresponds to the form function foo end and only creates a function without adding any methods.

args[2] - a SimpleVector of argument type data. args[2][1] is a Tuple type of the argument types, and args[2][2] is a SimpleVector of type variables corresponding to the method’s static parameters.

args[3] - a LambdaStaticData of the method itself.

args[4] - true or false, identifying whether the method is staged (@generated function)

const
declares a (global) variable as constant
null
has no arguments; simply yields the value nothing
static_typeof
a horrible misfeature used to determine the result type of array comprehensions. Planned to be removed.
type_goto
a virtual control flow edge used to convey type data to static_typeof, also to be removed.
new
allocates a new struct-like object. First argument is the type. The new pseudo-function is lowered to this, and the type is always inserted by the compiler. This is very much an internal-only feature, and does no checking. Evaluating arbitrary new expressions can easily segfault.
return
returns its argument as the value of the enclosing function.
the_exception
yields the caught exception inside a catch block. This is the value of the run time system variable jl_exception_in_transit.
enter
enters an exception handler (setjmp). args[1] is the label of the catch block to jump to on error.
leave
pop exception handlers. args[1] is the number of handlers to pop.
boundscheck
controls turning bounds checks on or off. A stack is maintained; if the first argument of this expression is true or false (true means bounds checks are enabled), it is pushed onto the stack. If the first argument is :pop, the stack is popped.
copyast
part of the implementation of quasi-quote. The argument is a surface syntax AST that is simply copied recursively and returned at run time.
meta
metadata. Currently used for inlining hints, represented by the symbols :inline and :noinline.

### LambdaStaticData¶

Has an ->ast field pointing to an Expr with head lambda. This Expr has the following layout:

args[1]
Vector{Any} of argument name symbols. For varargs functions, the last element is actually an Expr with head .... The argument of this Expr is an Expr with head ::. The first argument of :: is a symbol (the argument name), and the second argument is a type declaration.
args[2]

A Vector{Any} with variable information:

args[2][1] - An array of 3-element varinfo arrays, one for each argument or local variable. A varinfo array has the form Any[:name, type, bits]. The bits field is an integer describing variable properties as follows: - 1 - captured (closed over) - 2 - assigned (only false if there are no assignment statements with this var on the left) - 4 - assigned by an inner function - 8 - const (currently unused for local variables) - 16 - statically assigned once - 32 - might be used before assigned. This flag is only valid after type inference.

args[2][2] - An array of varinfo triples for each outer variable this function captures.

args[2][3] - The types of variables represented by GenSym objects. Given GenSym g, its type will be at args[2][3][g.id+1].

args[2][4] - The names (symbols) of static parameters.

args[3]
an Expr with head body whose arguments are the statements comprising the function body.

## Surface syntax AST¶

Front end ASTs consist entirely of Exprs and atoms (e.g. symbols, numbers). There is generally a different expression head for each visually distinct syntactic form. Examples will be given in s-expression syntax. Each parenthesized list corresponds to an Expr, where the first element is the head. For example (call f x) corresponds to Expr(:call, :f, :x) in julia.

### Calls¶

Input AST
f(x) (call f x)
f(x, y=1, z=2) (call f x (kw y 1) (kw z 2))
f(x; y=1) (call f (parameters (kw y 1)) x)
f(x...) (call f (... x))

Do syntax:

f(x) do a,b
body
end


parses as (call f (-> (tuple a b) (block body)) x).

### Operators¶

Most uses of operators are just function calls, so they are parsed with the head call. However some operators are special forms (not necessarily function calls), and in those cases the operator itself is the expression head. In julia-parser.scm these are referred to as “syntactic operators”. Some operators (+ and *) use N-ary parsing; chained calls are parsed as a single N-argument call. Finally, chains of comparisons have their own special expression structure.

Input AST
x+y (call + x y)
a+b+c+d (call + a b c d)
2x (call * 2 x)
a&&b (&& a b)
x += 1 (+= x 1)
a ? 1 : 2 (if a 1 2)
a:b (: a b)
a:b:c (: a b c)
a,b (tuple a b)
a==b (comparison a == b)
1<i<=n (comparison 1 < i <= n)
a.b (. a (quote b))
a.(b) (. a b)

### Bracketed forms¶

Input AST
a[i] (ref a i)
t[i;j] (typed_vcat t i j)
t[i j] (typed_hcat t i j)
t[a b; c d] (typed_vcat t (row a b) (row c d))
a{b} (curly a b)
a{b;c} (curly a (parameters c) b)
[x] (vect x)
[x,y] (vect x y)
[x;y] (vcat x y)
[x y] (hcat x y)
[x y; z t] (vcat (row x y) (row z t))
[x for y in z, a in b] (comprehension x (= y z) (= a b))
T[x for y in z] (typed_comprehension T x (= y z))
[a=>b for x in y] (dict_comprehension (=> a b) (= x y))
(k=>v)[a=>b for x in y] (typed_dict_comprehension (=> k v) (=> a b) (= x y))
(a, b, c) (tuple a b c)
(a; b; c) (block a (block b c))

### Macros¶

Input AST
@m x y (macrocall @m x y)
Base.@m x y (macrocall (. Base (quote @m)) x y)
@Base.m x y (macrocall (. Base (quote @m)) x y)

### Strings¶

Input AST
“a” “a”
x”y” (macrocall @x_str “y”)
x”y”z (macrocall @x_str “y” “z”)
“x = \$x” (string “x = ” x)
a b c (macrocall @cmd “a b c”)

Doc string syntax:

"some docs"
f(x) = x


parses as (macrocall (|.| Base '@doc) "some docs" (= (call f x) (block x)))

### Imports and such¶

Input AST
import a (import a)
import a.b.c (import a b c)
import ...a (import . . . a)
import a.b, c.d (toplevel (import a b) (import c d))
import Base: x (import Base x)
import Base: x, y (toplevel (import Base x) (import Base y))
export a, b (export a b)

### Numbers¶

Julia supports more number types than many scheme implementations, so not all numbers are represented directly as scheme numbers in the AST.

Input AST
11111111111111111111 (macrocall @int128_str “11111111111111111111”)
0xfffffffffffffffff (macrocall @uint128_str “0xfffffffffffffffff”)
1111...many digits... (macrocall @big_str “1111....”)

### Block forms¶

A block of statements is parsed as (block stmt1 stmt2 ...).

If statement:

if a
b
elseif c
d
else e
f
end


parses as:

(if a (block (line 2) b)
(block (line 3) (if c (block (line 4) d)
(block (line 5) e (line 6) f))))


A while loop parses as (while condition body).

A for loop parses as (for (= var iter) body). If there is more than one iteration specification, they are parsed as a block: (for (block (= v1 iter1) (= v2 iter2)) body).

break and continue are parsed as 0-argument expressions (break) and (continue).

let is parsed as (let body (= var1 val1) (= var2 val2) ...).

A basic function definition is parsed as (function (call f x) body). A more complex example:

function f{T}(x::T; k = 1)
return x+1
end


parses as:

(function (call (curly f T) (parameters (kw k 1))
(:: x T))
(block (line 2 file.jl) (return (call + x 1))))


Type definition:

type Foo{T<:S}
x::T
end


parses as:

(type #t (curly Foo (<: T S))
(block (line 2 none) (:: x T)))


The first argument is a boolean telling whether the type is mutable.

try blocks parse as (try try_block var catch_block finally_block). If no variable is present after catch, var is #f. If there is no finally clause, then the last argument is not present.