Metaprogramming¶
The strongest legacy of Lisp in the Julia language is its metaprogramming support. Like Lisp, Julia represents its own code as a data structure of the language itself. Since code is represented by objects that can be created and manipulated from within the language, it is possible for a program to transform and generate its own code. This allows sophisticated code generation without extra build steps, and also allows true Lisp-style macros operating at the level of abstract syntax trees. In contrast, preprocessor “macro” systems, like that of C and C++, perform textual manipulation and substitution before any actual parsing or interpretation occurs. Because all data types and code in Julia are represented by Julia data structures, powerful reflection capabilities are available to explore the internals of a program and its types just like any other data.
Program representation¶
Every Julia program starts life as a string:
julia> prog = "1 + 1"
"1 + 1"
What happens next?
The next step is to parse
each string into an object called an expression, represented by the Julia type
Expr
:
julia> ex1 = parse(prog)
:(1 + 1)
julia> typeof(ex1)
Expr
Expr
objects contain three parts:
- a
Symbol
identifying the kind of expression. A symbol is an interned string identifier (more discussion below).
julia> ex1.head
:call
- the expression arguments, which may be symbols, other expressions, or literal values:
julia> ex1.args
3-element Array{Any,1}:
:+
1
1
- finally, the expression result type, which may be annotated by the user or inferred by the compiler (and may be ignored completely for the purposes of this chapter):
julia> ex1.typ
Any
Expressions may also be constructed directly in prefix notation:
julia> ex2 = Expr(:call, :+, 1, 1)
:(1 + 1)
The two expressions constructed above – by parsing and by direct construction – are equivalent:
julia> ex1 == ex2
true
The key point here is that Julia code is internally represented as a data structure that is accessible from the language itself.
The dump()
function provides indented and annotated display of Expr
objects:
julia> dump(ex2)
Expr
head: Symbol call
args: Array(Any,(3,))
1: Symbol +
2: Int64 1
3: Int64 1
typ: Any
Expr
objects may also be nested:
julia> ex3 = parse("(4 + 4) / 2")
:((4 + 4) / 2)
Another way to view expressions is with Meta.show_sexpr, which displays the
S-expression form of a given
Expr
, which may look very familiar to users of Lisp. Here’s an example
illustrating the display on a nested Expr
:
julia> Meta.show_sexpr(ex3)
(:call, :/, (:call, :+, 4, 4), 2)
Symbols¶
The :
character has two syntactic purposes in Julia. The first form creates a
Symbol
, an interned string
used as one building-block of expressions:
julia> :foo
:foo
julia> typeof(ans)
Symbol
Symbol
s can also be created using symbol()
, which takes
a character or string as its argument:
julia> :foo == symbol("foo")
true
julia> symbol("'")
:'
In the context of an expression, symbols are used to indicate access to variables; when an expression is evaluated, a symbol is replaced with the value bound to that symbol in the appropriate scope.
Sometimes extra parentheses around the argument to :
are needed to avoid
ambiguity in parsing.:
julia> :(:)
:(:)
julia> :(::)
:(::)
Expressions and evaluation¶
Quoting¶
The second syntactic purpose of the :
character is to create expression
objects without using the explicit Expr
constructor. This is referred
to as quoting. The :
character, followed by paired parentheses around
a single statement of Julia code, produces an Expr
object based on the
enclosed code. Here is example of the short form used to quote an arithmetic
expression:
julia> ex = :(a+b*c+1)
:(a + b * c + 1)
julia> typeof(ex)
Expr
(to view the structure of this expression, try ex.head
and ex.args
,
or use dump()
as above)
Note that equivalent expressions may be constructed using parse()
or
the direct Expr
form:
julia> :(a + b*c + 1) ==
parse("a + b*c + 1") ==
Expr(:call, :+, :a, Expr(:call, :*, :b, :c), 1)
true
Expressions provided by the parser generally only have symbols, other
expressions, and literal values as their args, whereas expressions
constructed by Julia code can have arbitrary run-time values
without literal forms as args. In this specific example, +
and a
are symbols, *(b,c)
is a subexpression, and 1
is a literal
64-bit signed integer.
There is a second syntactic form of quoting for multiple expressions:
blocks of code enclosed in quote ... end
. Note that this form
introduces QuoteNode
elements to the expression tree, which
must be considered when directly manipulating an expression tree
generated from quote
blocks. For other purposes, :( ... )
and quote .. end
blocks are treated identically.
julia> ex = quote
x = 1
y = 2
x + y
end
quote # none, line 2:
x = 1 # line 3:
y = 2 # line 4:
x + y
end
julia> typeof(ex)
Expr
Interpolation¶
Direct construction of Expr
objects with value arguments is
powerful, but Expr
constructors can be tedious compared to “normal”
Julia syntax. As an alternative, Julia allows “splicing” or interpolation
of literals or expressions into quoted expressions. Interpolation is
indicated by the $
prefix.
In this example, the literal value of a is interpolated:
julia> a = 1;
julia> ex = :($a + b)
:(1 + b)
In this example, the tuple (1,2,3)
is interpolated as an
expression into a conditional test:
julia> ex = :(a in $:((1,2,3)) )
:($(Expr(:in, :a, :((1,2,3)))))
Interpolating symbols into a nested expression requires enclosing each symbol in an enclosing quote block:
julia> :( :a in $( :(:a + :b) ) )
^^^^^^^^^^
quoted inner expression
The use of $
for expression interpolation is intentionally reminiscent
of string interpolation and command
interpolation. Expression interpolation allows
convenient, readable programmatic construction of complex Julia expressions.
eval()
and effects¶
Given an expression object, one can cause Julia to evaluate (execute) it
at global scope using eval()
:
julia> :(1 + 2)
:(1 + 2)
julia> eval(ans)
3
julia> ex = :(c + d)
:(c + d)
julia> eval(ex)
ERROR: c not defined
julia> c = 1; d = 2;
julia> eval(ex)
3
Every module has its own eval()
function that
evaluates expressions in its global scope.
Expressions passed to eval()
are not limited to returning values
— they can also have side-effects that alter the state of the enclosing
module’s environment:
julia> ex = :(x = 1)
:(x = 1)
julia> x
ERROR: x not defined
julia> eval(ex)
1
julia> x
1
Here, the evaluation of an expression object causes a value to be
assigned to the global variable x
.
Since expressions are just Expr
objects which can be constructed
programmatically and then evaluated, it is possible to dynamically generate
arbitrary code which can then be run using eval()
. Here is a simple example:
julia> a = 1;
julia> ex = Expr(:call, :+, a, :b)
:(1 + b)
julia> a = 0; b = 2;
julia> eval(ex)
3
The value of a
is used to construct the expression ex
which
applies the +
function to the value 1 and the variable b
. Note
the important distinction between the way a
and b
are used:
- The value of the variable
a
at expression construction time is used as an immediate value in the expression. Thus, the value ofa
when the expression is evaluated no longer matters: the value in the expression is already1
, independent of whatever the value ofa
might be. - On the other hand, the symbol
:b
is used in the expression construction, so the value of the variableb
at that time is irrelevant —:b
is just a symbol and the variableb
need not even be defined. At expression evaluation time, however, the value of the symbol:b
is resolved by looking up the value of the variableb
.
Functions on Expr
essions¶
As hinted above, one extremely useful feature of Julia is the capability to
generate and manipulate Julia code within Julia itself. We have already
seen one example of a function returning Expr
objects: the parse()
function, which takes a string of Julia code and returns the corresponding
Expr
. A function can also take one or more Expr
objects as
arguments, and return another Expr
. Here is a simple, motivating example:
julia> function math_expr(op, op1, op2)
expr = Expr(:call, op, op1, op2)
return expr
end
julia> ex = math_expr(:+, 1, Expr(:call, :*, 4, 5))
:(1 + 4*5)
julia> eval(ex)
21
As another example, here is a function that doubles any numeric argument, but leaves expressions alone:
julia> function make_expr2(op, opr1, opr2)
opr1f, opr2f = map(x -> isa(x, Number) ? 2*x : x, (opr1, opr2))
retexpr = Expr(:call, op, opr1f, opr2f)
return retexpr
end
make_expr2 (generic function with 1 method)
julia> make_expr2(:+, 1, 2)
:(2 + 4)
julia> ex = make_expr2(:+, 1, Expr(:call, :*, 5, 8))
:(2 + 5 * 8)
julia> eval(ex)
42
Macros¶
Macros provide a method to include generated code in the final body
of a program. A macro maps a tuple of arguments to a returned
expression, and the resulting expression is compiled directly rather
than requiring a runtime eval()
call. Macro arguments may include
expressions, literal values, and symbols.
Basics¶
Here is an extraordinarily simple macro:
julia> macro sayhello()
return :( println("Hello, world!") )
end
Macros have a dedicated character in Julia’s syntax: the @
(at-sign),
followed by the unique name declared in a macro NAME ... end
block.
In this example, the compiler will replace all instances of @sayhello
with:
:( println("Hello, world!") )
When @sayhello
is given at the REPL, the expression executes
immediately, thus we only see the evaluation result:
julia> @sayhello()
"Hello, world!"
Now, consider a slightly more complex macro:
julia> macro sayhello(name)
return :( println("Hello, ", $name) )
end
This macro takes one argument: name
. When @sayhello
is
encountered, the quoted expression is expanded to interpolate
the value of the argument into the final expression:
julia> @sayhello("human")
Hello, human
We can view the quoted return expression using the function macroexpand()
(important note: this is an extremely useful tool for debugging macros):
julia> ex = macroexpand( :(@sayhello("human")) )
:(println("Hello, ","human"))
^^^^^^^
interpolated: now a literal string
julia> typeof(ex)
Expr
Hold up: why macros?¶
We have already seen a function f(::Expr...) -> Expr
in a previous section.
In fact, macroexpand()
is also such a function. So, why do macros
exist?
Macros are necessary because they execute when code is parsed, therefore, macros allow the programmer to generate and include fragments of customized code before the full program is run. To illustrate the difference, consider the following example:
julia> macro twostep(arg)
println("I execute at parse time. The argument is: ", arg)
return :(println("I execute at runtime. The argument is: ", $arg))
end
julia> ex = macroexpand( :(@twostep :(1, 2, 3)) );
I execute at parse time. The argument is: :((1,2,3))
The first call to println()
is executed when macroexpand()
is called. The resulting expression contains only the second println
:
julia> typeof(ex)
Expr
julia> ex
:(println("I execute at runtime. The argument is: ",$(Expr(:copyast, :(:((1,2,3)))))))
julia> eval(ex)
I execute at runtime. The argument is: (1,2,3)
Macro invocation¶
Macros are invoked with the following general syntax:
@name expr1 expr2 ...
@name(expr1, expr2, ...)
Note the distinguishing @
before the macro name and the lack of
commas between the argument expressions in the first form, and the
lack of whitespace after @name
in the second form. The two styles
should not be mixed. For example, the following syntax is different
from the examples above; it passes the tuple (expr1, expr2, ...)
as
one argument to the macro:
@name (expr1, expr2, ...)
It is important to emphasize that macros receive their arguments as
expressions, literals, or symbols. One way to explore macro arguments
is to call the show()
function within the macro body:
julia> macro showarg(x)
show(x)
# ... remainder of macro, returning an expression
end
julia> @showarg(a)
(:a,)
julia> @showarg(1+1)
:(1 + 1)
julia> @showarg(println("Yo!")
:(println("Yo!"))
Building an advanced macro¶
Here is a simplified definition of Julia’s @assert
macro:
macro assert(ex)
return :($ex ? nothing : error("Assertion failed: ", $(string(ex))))
end
This macro can be used like this:
julia> @assert 1==1.0
julia> @assert 1==0
ERROR: assertion failed: 1 == 0
in error at error.jl:21
In place of the written syntax, the macro call is expanded at parse time to its returned result. This is equivalent to writing:
1==1.0 ? nothing : error("Assertion failed: ", "1==1.0")
1==0 ? nothing : error("Assertion failed: ", "1==0")
That is, in the first call, the expression :(1==1.0)
is spliced into
the test condition slot, while the value of string(:(1==1.0))
is
spliced into the assertion message slot. The entire expression, thus
constructed, is placed into the syntax tree where the @assert
macro
call occurs. Then at execution time, if the test expression evaluates to
true, then nothing
is returned, whereas if the test is false, an error
is raised indicating the asserted expression that was false. Notice that
it would not be possible to write this as a function, since only the
value of the condition is available and it would be impossible to
display the expression that computed it in the error message.
The actual definition of @assert
in the standard library is more
complicated. It allows the user to optionally specify their own error
message, instead of just printing the failed expression. Just like in
functions with a variable number of arguments, this is specified with an
ellipses following the last argument:
macro assert(ex, msgs...)
msg_body = isempty(msgs) ? ex : msgs[1]
msg = string("assertion failed: ", msg_body)
return :($ex ? nothing : error($msg))
end
Now @assert
has two modes of operation, depending upon the number of
arguments it receives! If there’s only one argument, the tuple of expressions
captured by msgs
will be empty and it will behave the same as the simpler
definition above. But now if the user specifies a second argument, it is
printed in the message body instead of the failing expression. You can inspect
the result of a macro expansion with the aptly named macroexpand()
function:
julia> macroexpand(:(@assert a==b))
:(if a == b
nothing
else
Base.error("assertion failed: a == b")
end)
julia> macroexpand(:(@assert a==b "a should equal b!"))
:(if a == b
nothing
else
Base.error("assertion failed: a should equal b!")
end)
There is yet another case that the actual @assert
macro handles: what
if, in addition to printing “a should equal b,” we wanted to print their
values? One might naively try to use string interpolation in the custom
message, e.g., @assert a==b "a ($a) should equal b ($b)!"
, but this
won’t work as expected with the above macro. Can you see why? Recall
from string interpolation that an
interpolated string is rewritten to a call to string()
.
Compare:
julia> typeof(:("a should equal b"))
ASCIIString (constructor with 2 methods)
julia> typeof(:("a ($a) should equal b ($b)!"))
Expr
julia> dump(:("a ($a) should equal b ($b)!"))
Expr
head: Symbol string
args: Array(Any,(5,))
1: ASCIIString "a ("
2: Symbol a
3: ASCIIString ") should equal b ("
4: Symbol b
5: ASCIIString ")!"
typ: Any
So now instead of getting a plain string in msg_body
, the macro is
receiving a full expression that will need to be evaluated in order to
display as expected. This can be spliced directly into the returned expression
as an argument to the string()
call; see error.jl for
the complete implementation.
The @assert
macro makes great use of splicing into quoted expressions
to simplify the manipulation of expressions inside the macro body.
Hygiene¶
An issue that arises in more complex macros is that of
hygiene. In short, macros must
ensure that the variables they introduce in their returned expressions do not
accidentally clash with existing variables in the surrounding code they expand
into. Conversely, the expressions that are passed into a macro as arguments are
often expected to evaluate in the context of the surrounding code,
interacting with and modifying the existing variables. Another concern arises
from the fact that a macro may be called in a different module from where it
was defined. In this case we need to ensure that all global variables are
resolved to the correct module. Julia already has a major advantage over
languages with textual macro expansion (like C) in that it only needs to
consider the returned expression. All the other variables (such as msg
in
@assert
above) follow the normal scoping block behavior.
To demonstrate these issues,
let us consider writing a @time
macro that takes an expression as
its argument, records the time, evaluates the expression, records the
time again, prints the difference between the before and after times,
and then has the value of the expression as its final value.
The macro might look like this:
macro time(ex)
return quote
local t0 = time()
local val = $ex
local t1 = time()
println("elapsed time: ", t1-t0, " seconds")
val
end
end
Here, we want t0
, t1
, and val
to be private temporary variables,
and we want time
to refer to the time()
function in the standard library,
not to any time
variable the user might have (the same applies to
println
). Imagine the problems that could occur if the user expression
ex
also contained assignments to a variable called t0
, or defined
its own time
variable. We might get errors, or mysteriously incorrect
behavior.
Julia’s macro expander solves these problems in the following way. First,
variables within a macro result are classified as either local or global.
A variable is considered local if it is assigned to (and not declared
global), declared local, or used as a function argument name. Otherwise,
it is considered global. Local variables are then renamed to be unique
(using the gensym()
function, which generates new symbols), and global
variables are resolved within the macro definition environment. Therefore
both of the above concerns are handled; the macro’s locals will not conflict
with any user variables, and time
and println
will refer to the
standard library definitions.
One problem remains however. Consider the following use of this macro:
module MyModule
import Base.@time
time() = ... # compute something
@time time()
end
Here the user expression ex
is a call to time
, but not the same
time
function that the macro uses. It clearly refers to MyModule.time
.
Therefore we must arrange for the code in ex
to be resolved in the
macro call environment. This is done by “escaping” the expression with
esc()
:
macro time(ex)
...
local val = $(esc(ex))
...
end
An expression wrapped in this manner is left alone by the macro expander and simply pasted into the output verbatim. Therefore it will be resolved in the macro call environment.
This escaping mechanism can be used to “violate” hygiene when necessary,
in order to introduce or manipulate user variables. For example, the
following macro sets x
to zero in the call environment:
macro zerox()
return esc(:(x = 0))
end
function foo()
x = 1
@zerox
x # is zero
end
This kind of manipulation of variables should be used judiciously, but is occasionally quite handy.
Code Generation¶
When a significant amount of repetitive boilerplate code is required, it
is common to generate it programmatically to avoid redundancy. In most
languages, this requires an extra build step, and a separate program to
generate the repetitive code. In Julia, expression interpolation and
eval()
allow such code generation to take place in the normal course of
program execution. For example, the following code defines a series of
operators on three arguments in terms of their 2-argument forms:
for op = (:+, :*, :&, :|, :$)
eval(quote
($op)(a,b,c) = ($op)(($op)(a,b),c)
end)
end
In this manner, Julia acts as its own preprocessor, and allows code
generation from inside the language. The above code could be written
slightly more tersely using the :
prefix quoting form:
for op = (:+, :*, :&, :|, :$)
eval(:(($op)(a,b,c) = ($op)(($op)(a,b),c)))
end
This sort of in-language code generation, however, using the
eval(quote(...))
pattern, is common enough that Julia comes with a
macro to abbreviate this pattern:
for op = (:+, :*, :&, :|, :$)
@eval ($op)(a,b,c) = ($op)(($op)(a,b),c)
end
The @eval
macro rewrites this call to be precisely equivalent to the
above longer versions. For longer blocks of generated code, the
expression argument given to @eval
can be a block:
@eval begin
# multiple lines
end
Interpolating into an unquoted expression is not supported and will cause a compile-time error:
julia> $a + b
ERROR: unsupported or misplaced expression $
Non-Standard String Literals¶
Recall from Strings that string literals prefixed by an identifier are called non-standard string literals, and can have different semantics than un-prefixed string literals. For example:
r"^\s*(?:#|$)"
produces a regular expression object rather than a stringb"DATA\xff\u2200"
is a byte array literal for[68,65,84,65,255,226,136,128]
.
Perhaps surprisingly, these behaviors are not hard-coded into the Julia parser or compiler. Instead, they are custom behaviors provided by a general mechanism that anyone can use: prefixed string literals are parsed as calls to specially-named macros. For example, the regular expression macro is just the following:
macro r_str(p)
Regex(p)
end
That’s all. This macro says that the literal contents of the string
literal r"^\s*(?:#|$)"
should be passed to the @r_str
macro and
the result of that expansion should be placed in the syntax tree where
the string literal occurs. In other words, the expression
r"^\s*(?:#|$)"
is equivalent to placing the following object
directly into the syntax tree:
Regex("^\\s*(?:#|\$)")
Not only is the string literal form shorter and far more convenient, but
it is also more efficient: since the regular expression is compiled and
the Regex
object is actually created when the code is compiled,
the compilation occurs only once, rather than every time the code is
executed. Consider if the regular expression occurs in a loop:
for line = lines
m = match(r"^\s*(?:#|$)", line)
if m == nothing
# non-comment
else
# comment
end
end
Since the regular expression r"^\s*(?:#|$)"
is compiled and inserted
into the syntax tree when this code is parsed, the expression is only
compiled once instead of each time the loop is executed. In order to
accomplish this without macros, one would have to write this loop like
this:
re = Regex("^\\s*(?:#|\$)")
for line = lines
m = match(re, line)
if m == nothing
# non-comment
else
# comment
end
end
Moreover, if the compiler could not determine that the regex object was constant over all loops, certain optimizations might not be possible, making this version still less efficient than the more convenient literal form above. Of course, there are still situations where the non-literal form is more convenient: if one needs to interpolate a variable into the regular expression, one must take this more verbose approach; in cases where the regular expression pattern itself is dynamic, potentially changing upon each loop iteration, a new regular expression object must be constructed on each iteration. In the vast majority of use cases, however, regular expressions are not constructed based on run-time data. In this majority of cases, the ability to write regular expressions as compile-time values is invaluable.
The mechanism for user-defined string literals is deeply, profoundly
powerful. Not only are Julia’s non-standard literals implemented using
it, but also the command literal syntax (`echo "Hello, $person"`
)
is implemented with the following innocuous-looking macro:
macro cmd(str)
:(cmd_gen($shell_parse(str)))
end
Of course, a large amount of complexity is hidden in the functions used in this macro definition, but they are just functions, written entirely in Julia. You can read their source and see precisely what they do — and all they do is construct expression objects to be inserted into your program’s syntax tree.