.. _man-metaprogramming: .. currentmodule:: Base ***************** 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: .. doctest:: 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 :obj:`Expr`: .. doctest:: julia> ex1 = parse(prog) :(1 + 1) julia> typeof(ex1) Expr :obj:`Expr` objects contain three parts: - a ``Symbol`` identifying the kind of expression. A symbol is an `interned string `_ identifier (more discussion below). .. doctest:: julia> ex1.head :call - the expression arguments, which may be symbols, other expressions, or literal values: .. doctest:: 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): .. doctest:: julia> ex1.typ Any Expressions may also be constructed directly in `prefix notation `_: .. doctest:: julia> ex2 = Expr(:call, :+, 1, 1) :(1 + 1) The two expressions constructed above -- by parsing and by direct construction -- are equivalent: .. doctest:: 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 :func:`dump` function provides indented and annotated display of :obj:`Expr` objects: .. doctest:: julia> dump(ex2) Expr head: Symbol call args: Array(Any,(3,)) 1: Symbol + 2: Int64 1 3: Int64 1 typ: Any :obj:`Expr` objects may also be nested: .. doctest:: 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 :obj:`Expr`, which may look very familiar to users of Lisp. Here's an example illustrating the display on a nested :obj:`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 :obj:`Symbol`, an `interned string `_ used as one building-block of expressions: .. doctest:: julia> :foo :foo julia> typeof(ans) Symbol :obj:`Symbol`\ s can also be created using :func:`symbol`, which takes a character or string as its argument: .. doctest:: 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 :ref:`scope `. Sometimes extra parentheses around the argument to ``:`` are needed to avoid ambiguity in parsing.: .. doctest:: julia> :(:) :(:) julia> :(::) :(::) Expressions and evaluation -------------------------- Quoting ~~~~~~~ The second syntactic purpose of the ``:`` character is to create expression objects without using the explicit :obj:`Expr` constructor. This is referred to as *quoting*. The ``:`` character, followed by paired parentheses around a single statement of Julia code, produces an :obj:`Expr` object based on the enclosed code. Here is example of the short form used to quote an arithmetic expression: .. doctest:: 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 :func:`dump` as above) Note that equivalent expressions may be constructed using :func:`parse` or the direct :obj:`Expr` form: .. doctest:: 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 :obj:`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. .. doctest:: 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 :obj:`Expr` objects with value arguments is powerful, but :obj:`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 ``$`` prefix. In this example, the literal value of `a` is interpolated: .. doctest:: julia> a = 1; julia> ex = :($a + b) :(1 + b) In this example, the tuple ``(1,2,3)`` is interpolated as an expression into a conditional test: .. doctest:: 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 :ref:`string interpolation ` and :ref:`command interpolation `. Expression interpolation allows convenient, readable programmatic construction of complex Julia expressions. :func:`eval` and effects ~~~~~~~~~~~~~~~~~~~~~~~~ Given an expression object, one can cause Julia to evaluate (execute) it at global scope using :func:`eval`: .. doctest:: 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 :ref:`module ` has its own :func:`eval` function that evaluates expressions in its global scope. Expressions passed to :func:`eval` are not limited to returning values — they can also have side-effects that alter the state of the enclosing module's environment: .. doctest:: 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 :obj:`Expr` objects which can be constructed programmatically and then evaluated, it is possible to dynamically generate arbitrary code which can then be run using :func:`eval`. Here is a simple example: .. doctest:: 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 of ``a`` when the expression is evaluated no longer matters: the value in the expression is already ``1``, independent of whatever the value of ``a`` might be. - On the other hand, the *symbol* ``:b`` is used in the expression construction, so the value of the variable ``b`` at that time is irrelevant — ``:b`` is just a symbol and the variable ``b`` 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 variable ``b``. Functions on :obj:`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 :obj:`Expr` objects: the :func:`parse` function, which takes a string of Julia code and returns the corresponding :obj:`Expr`. A function can also take one or more :obj:`Expr` objects as arguments, and return another :obj:`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 :func:`eval` call. Macro arguments may include expressions, literal values, and symbols. Basics ~~~~~~ Here is an extraordinarily simple macro: .. doctest:: 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 :func:`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, :func:`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: ", $arg)) end julia> ex = macroexpand( :(@twostep :(1, 2, 3)) ); I execute at parse time. The argument is: :((1,2,3)) The first call to :func:`println` is executed when :func:`macroexpand` is called. The resulting expression contains *only* the second ``println``:: julia> typeof(ex) Expr julia> ex :(println("I execute at runtime. The argument is: ",$(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 :func:`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 :obj:`@assert` macro:: macro assert(ex) return :($ex ? nothing : error("Assertion failed: ", $(string(ex)))) end This macro can be used like this: .. doctest:: julia> @assert 1==1.0 julia> @assert 1==0 ERROR: assertion failed: 1 == 0 in error at error.jl:21 In place of the written syntax, the macro call is expanded at parse time to its returned result. This is equivalent to writing:: 1==1.0 ? nothing : error("Assertion failed: ", "1==1.0") 1==0 ? nothing : error("Assertion failed: ", "1==0") That is, in the first call, the expression ``:(1==1.0)`` is spliced into the test condition slot, while the value of ``string(:(1==1.0))`` is spliced into the assertion message slot. The entire expression, thus constructed, is placed into the syntax tree where the :obj:`@assert` macro call occurs. Then at execution time, if the test expression evaluates to true, then ``nothing`` is returned, whereas if the test is false, an error is raised indicating the asserted expression that was false. Notice that it would not be possible to write this as a function, since only the *value* of the condition is available and it would be impossible to display the expression that computed it in the error message. The actual definition of :obj:`@assert` in the standard library is more complicated. It allows the user to optionally specify their own error message, instead of just printing the failed expression. Just like in functions with a variable number of arguments, this is specified with an ellipses following the last argument:: macro assert(ex, msgs...) msg_body = isempty(msgs) ? ex : msgs[1] msg = string("assertion failed: ", msg_body) return :($ex ? nothing : error($msg)) end Now :obj:`@assert` has two modes of operation, depending upon the number of arguments it receives! If there's only one argument, the tuple of expressions captured by ``msgs`` will be empty and it will behave the same as the simpler definition above. But now if the user specifies a second argument, it is printed in the message body instead of the failing expression. You can inspect the result of a macro expansion with the aptly named :func:`macroexpand` function: .. doctest:: julia> macroexpand(:(@assert a==b)) :(if a == b nothing else Base.error("assertion failed: a == b") end) julia> macroexpand(:(@assert a==b "a should equal b!")) :(if a == b nothing else Base.error("assertion failed: a should equal b!") end) There is yet another case that the actual :obj:`@assert` macro handles: what if, in addition to printing "a should equal b," we wanted to print their values? One might naively try to use string interpolation in the custom message, e.g., ``@assert a==b "a ($a) should equal b ($b)!"``, but this won't work as expected with the above macro. Can you see why? Recall from :ref:`string interpolation ` that an interpolated string is rewritten to a call to :func:`string`. Compare: .. doctest:: julia> typeof(:("a should equal b")) ASCIIString (constructor with 2 methods) julia> typeof(:("a ($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 :func:`string` call; see `error.jl `_ for the complete implementation. The :obj:`@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 :obj:`@assert` above) follow the :ref:`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 :func:`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 :func:`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 :func:`esc`:: macro time(ex) ... local val = $(esc(ex)) ... end An expression wrapped in this manner is left alone by the macro expander and simply pasted into the output verbatim. Therefore it will be resolved in the macro call environment. This escaping mechanism can be used to "violate" hygiene when necessary, in order to introduce or manipulate user variables. For example, the following macro sets ``x`` to zero in the call environment:: macro zerox() return esc(:(x = 0)) end function foo() x = 1 @zerox x # is zero end This kind of manipulation of variables should be used judiciously, but is occasionally quite handy. .. _man-non-standard-string-literals2: Code Generation --------------- When a significant amount of repetitive boilerplate code is required, it is common to generate it programmatically to avoid redundancy. In most languages, this requires an extra build step, and a separate program to generate the repetitive code. In Julia, expression interpolation and :func:`eval` allow such code generation to take place in the normal course of program execution. For example, the following code defines a series of operators on three arguments in terms of their 2-argument forms:: for op = (:+, :*, :&, :|, :$) 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 :obj:`@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 :obj:`@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: .. doctest:: julia> $a + b ERROR: unsupported or misplaced expression $ .. _man-macros: Non-Standard String Literals ---------------------------- Recall from :ref:`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*(?:#|$)"`` produces a regular expression object rather than a string - ``b"DATA\xff\u2200"`` is a byte array literal for ``[68,65,84,65,255,226,136,128]``. Perhaps surprisingly, these behaviors are not hard-coded into the Julia parser or compiler. Instead, they are custom behaviors provided by a general mechanism that anyone can use: prefixed string literals are parsed as calls to specially-named macros. For example, the regular expression macro is just the following:: macro r_str(p) Regex(p) end That's all. This macro says that the literal contents of the string literal ``r"^\s*(?:#|$)"`` 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*(?:#|$)"`` is equivalent to placing the following object directly into the syntax tree:: Regex("^\\s*(?:#|\$)") Not only is the string literal form shorter and far more convenient, but it is also more efficient: since the regular expression is compiled and the :obj:`Regex` object is actually created *when the code is compiled*, the compilation occurs only once, rather than every time the code is executed. Consider if the regular expression occurs in a loop:: for line = lines m = match(r"^\s*(?:#|$)", 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.