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The `:` character

The : character has two syntatic purposes in Julia. It signifies a Symbol or a Quote.

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Interpolation

Julia allows interpolation of literals or expressions into quoted expressions by prefixing a variable with the $ character. For example.

begin
a = 1;
ex5 = :($a + b)
dump(ex5)
end

Try it.

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Usage

Macros can be used for many tasks such as performing operations on blocks of code, e.g. timing expressions (@time), threading for loops (@threads), and converting expressions to broadcast form (@.). Another use is to generate code. When a significant amount of repetitive boilerplate code is required, it is common to generate it programmatically to avoid redundancy.

Consider the following example.

struct MyNumber
    x::Float64
end

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So, there are three ways to construct an expression allowing the programmer to use whichever one is most convenient.

There is a second syntatic form of quoting, called a quote block (i.e. quote ... end), that is commonly used for multiple expressions.

Try the following example.

begin
ex4 = quote
    x = 1
    y = 2
    x + y
end
println(typeof(ex4))
end

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Expression evaluation

Julia will evalution an expression in the global scope using eval.

println(eval(ex1))

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begin
x = foo(2)
x
end

Note

The result of the function call is the return type, not the return value.

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Splatting is also possible.

begin
args = [:x, :y, :x];
dump(:(f(1, $(args...))))
end

And this too.

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begin
y = foo("bar")
y
end

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A slightly shorter version of the above code generator that use the : prefix is:

for op = (:sin, :cos, :tan, :log, :exp)
    eval(:(Base.$op(a::MyNumber) = MyNumber($op(a.x))))
end

An even shorter version of the code uses the @eval macro.

for op = (:sin, :cos, :tan, :log, :exp)
    @eval Base.$op(a::MyNumber) = MyNumber($op(a.x))
end

For longer blocks of generated code, the @eval macro can proceed a code block:

@eval begin
    # multiple lines
end

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Generated functions differ from regular functions in five ways. They:

are prefixed by @generated. This provides the AST with some additional information.
can only access the types of the arguments, not their values.
are nonexecuting. They return a quoted expression, not a value.
can only call previously defined functions. (Otherwise, MethodErrors may result.)
must not mutate or observe any non-constant global state. They can only read global constants, and cannot have side effects. In other words, they must be completely pure.

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Now invoke the @sayhello macro.

@sayhello("world")

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Basics

Defining a macro is like defining a function. For example,

macro sayhello(name)
    return :( println("Hello", $name))
end

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The Abstract Syntax Tree

begin
str1 = "3 * 4 + 5"
ex1 = Meta.parse(str1)
println(typeof(ex1))
end

Try it.

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Note that the parsed expression is an Expr type.

dump(ex1)

Show or dump the abstract syntax tree (AST) of the expression.

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Generated Functions

A special macro is @generated. It defines so-called generated functions. They have the capability to generate specialized code depending on the types of their arguments with more flexibility and/or less code than what can be done with multiple dispatch. Macros work with expressions at the AST level and cannot acces the input types. Whereas, generated functions are expanded at the time when the argument types are known, but before the function is compiled.

Here is an example.

@generated function foo(x)
    Core.println(x)
    return :(x * x)
end

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Quoting

The second purpose of the : character is to create expression objects without using the explicit Expr constructor. This is referred to as quoting.

begin
ex3 = :(3 * 4 + 5)
dump(ex3)
println(ex1 == ex2 == ex3)
end

Try this example.

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Parentheses around the arguments are optional.

@sayhello "world"

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The Symbol constructor takes a variable number of arguments and concatenates them together to create a Symbol string.

Symbol("func", 10)

Try this.

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Macros

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Note

When using the parenthesized form, there should be no space between the macro name and the parentheses. Otherwise the argument list will be interpreted as a single argument containing a tuple.

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The Expr type has two fields, a head and an args. The head declares the item is a function call of type Symbol and the args is an Array of function arguments, namely the + operator of type Symbol and the two values to be added. The first value is another Expr type and the second is a 64-bit integer. The second nested Expr is also composed of a Symbol call. Its arguments are the multiplication * operation of type Symbol and two 64-bit integers. Note that the multiplication operation is nested in the addition operation, since multiplication has precedence over addition.

Note that the expression ex1 can be written directly in AST syntax.

begin
ex2 = Expr(:call, :+, Expr(:call, :*, 3, 4), 5)
dump(ex2)
println(ex1 == ex2)
end

Try it.

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We want to add a various methods to it. This can be done programmatically using a loop.

begin
println(length(methods(sin)))
for op = (:sin, :tan, :log, :exp)
    eval(quote
        Base.$op(a::MyNumber) = MyNumber($op(a.x))
    end)
end
println(length(methods(sin)))
end

Try this example.

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Unlike Matlab, Python, and R, Julia has macros. They are a very powerful programming construct. Macros change existing source code or generate entirely new code. In essence, they simplify programming by automating mundane coding tasks.

Preprocessor "macro" systems, like that of C/C++, work at the source code level by perform textual manipulation and substitution before any parsing or interpretation is done. Whereas, Julia macros work at the level of the abstract syntax tree after parsing, but before compilation is done.

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In the context of expressions, 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.

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Note

If you prefer writing code as an AST, you can do so in Julia.

However, you may not be popular with other coders.

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Using Macros

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Symbols

A Symbol is an interned string used as a building-block of an expression.

begin
sym = :foo
println(typeof(sym))
println(sym == Symbol("foo"))
end

Try it.

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The : character

Interpolation

Usage

Expression evaluation

Basics

The Abstract Syntax Tree

Generated Functions

Quoting

Macros

Using Macros

Symbols

The `:` character