https://github.com/JuliaLang/julia
Tip revision: 78d25695b9d8aff86517e30d927330015c7b8573 authored by Yichao Yu on 18 July 2015, 16:46:28 UTC
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basedocs.jl
# This file is a part of Julia. License is MIT: http://julialang.org/license
module BaseDocs
immutable Keyword
name :: Symbol
end
macro kw_str(text) Keyword(Symbol(text)) end
"Hello, Human."
kw"hello", kw"hi"
"""
**Welcome to Julia $(string(VERSION)).** The full manual is available at
http://docs.julialang.org/
as well many great tutorials and learning resources:
http://julialang.org/learning/
For help on a specific function or macro, type `?` followed
by its name, e.g. `?fft`, or `?@time`, and press enter.
"""
kw"help", kw"?", kw"julia"
"""
`using` will load the given module or package and make some of its names available for
use (see also `export`). For example:
using Gadfly
loads the plotting package, Gadfly, so that the `plot` function can be used.
Names can be used via dot syntax, whether they are exported or not:
Gadfly.plot(...)
If you don't want to use the packages exports directly, see also `import`. If you're not
sure, `using` is almost definitely what you want.
"""
kw"using"
"""
import Gadfly
`import`, like `using`, will load modules and packages for use. Unlike `using`, however,
it will *not* make any `export`ed names available for use. To use Gadfly's `plot`
function after importing it, for example, you have to write:
Gadfly.plot(...)
Import can also be used with specific names, for example
import Gadfly: plot, render
This syntax is used when you want to extend the modules functions with new methods.
"""
kw"import"
"""
`export` is used within modules and packages to tell Julia which functions should be
made available to the user. For example:
module Test
export foo # foo is exported, but bar isn't
foo(x) = x
bar(y) = y
end
using Test
foo(1) # 1
bar(1) # Error: bar not defined
Test.bar(1) # 1
"""
kw"export"
"""
`abstract` declares a type that cannot be instantiated, and serves only as a node in the
type graph, thereby describing sets of related concrete types: those concrete types
which are their descendants. Abstract types form the conceptual hierarchy which makes
Julia’s type system more than just a collection of object implementations. For example:
abstract Number
abstract Real <: Number
`abstract Number` has no supertype, whereas `abstract Real` is an abstract subtype of `Number`.
"""
kw"abstract"
"""
`module` declares a Module, which is a separate global variable workspace. Within a
module, you can control which names from other modules are visible (via importing), and
specify which of your names are intended to be public (via exporting). For example:
module
import Base.show
export MyType, foo
type MyType
x
end
bar(x) = 2x
foo(a::MyType) = bar(a.x) + 1
show(io, a::MyType) = print(io, "MyType \$(a.x)")
end
Modules allow you to create top-level definitions without worrying about name conflicts
when your code is used together with somebody else’s.
"""
kw"module"
"""
`baremodule` declares a module that does not contain `using Base`
or a definition of `eval`. It does still import `Core`.
"""
kw"baremodule"
"""
`bitstype` declares a concrete type whose data consists of plain old bits. Classic
examples of bits types are integers and floating-point values. Some example built-in
bits type declarations:
bitstype 32 Char
bitstype 8 Bool <: Integer
The first parameter indicates how many bits of storage the type requires. Currently,
only sizes that are multiples of 8 bits are supported. The second parameter gives the
name of the type. The `Bool` declaration shows how a bits type can be optionally
declared to be a subtype of some supertype.
"""
kw"bitstype"
"""
`macro` defines 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. For example:
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.
"""
kw"macro"
"""
`importall` imports all names exported by the specified module, as if `import` were used
individually on all of them. For example:
importall Distributions
As with `import`, functions imported by `importall` can be extended.
"""
kw"importall"
"""
`local` introduces a new local variable. For example:
function foo(n)
x = 0
for i = 1:n
local x
x = i
end
x
end
julia> foo(10)
0
Here `local x` introduces a separate `x` inside the loop, so the function returns `0`.
"""
kw"local"
"""
`global x` makes `x` in the current scope and its inner scopes refer to the global
variable of that name. In the example below, `global` is needed so the function can
modify the global variable `z`:
z=3
function foo()
global z=6
end
julia> foo()
6
julia> z
6
Without the `global` declaration in `foo()`, a new local variable would have been
created inside foo(), and the `z` in the global scope would have remained equal to `3`.
"""
kw"global"
"""
`let` statements allocate new variable bindings each time they run. Whereas an
assignment modifies an existing value location, `let` creates new locations. This
difference is only detectable in the case of variables that outlive their scope via
closures. The `let` syntax accepts a comma-separated series of assignments and variable
names:
let var1 = value1, var2, var3 = value3
code
end
The assignments are evaluated in order, with each right-hand side evaluated in the scope
before the new variable on the left-hand side has been introduced. Therefore it makes
sense to write something like `let x = x`, since the two `x` variables are distinct and
have separate storage.
"""
kw"let"
"""
`quote` creates multiple expression objects in a block without using the explicit `Expr`
constructor. For example:
ex = quote
x = 1
y = 2
x + y
end
Unlike the other means of quoting, `:( ... )`, this form introduces `QuoteNode` elements
to the expression tree, which must be considered when directly manipulating the tree.
For other purposes, `:( ... )` and `quote .. end` blocks are treated identically.
"""
kw"quote"
"""
`'` is the conjugate transposition operator:
julia> A = reshape(1:4, 2,2)
2×2 Array{Int64,2}:
1 3
2 4
julia> A'
2×2 Array{Int64,2}:
1 2
3 4
julia> B = A + im
2×2 Array{Complex{Int64},2}:
1+1im 3+1im
2+1im 4+1im
julia> B'
2×2 Array{Complex{Int64},2}:
1-1im 2-1im
3-1im 4-1im
"""
kw"'"
"""
`.'` is the transposition operator:
julia> A = reshape(1:4, 2,2)
2×2 Array{Int64,2}:
1 3
2 4
julia> A.'
2×2 Array{Int64,2}:
1 2
3 4
julia> B = A + im
2×2 Array{Complex{Int64},2}:
1+1im 3+1im
2+1im 4+1im
julia> B.'
2×2 Array{Complex{Int64},2}:
1+1im 2+1im
3+1im 4+1im
"""
kw".'"
"""
`const` is used to declare global variables which are also constant. In almost all code
(and particularly performance sensitive code) global variables should be declared
constant in this way.
const x = 5
Note that "constant-ness" is not enforced inside containers, so if `x` is an array or
dictionary (for example) you can still add and remove elements.
Technically, you can even redefine `const` variables, although this will generate a
warning from the compiler. The only strict requirement is that the *type* of the
variable does not change, which is why `const` variables are much faster than regular
globals.
"""
kw"const"
"""
Functions are defined with the `function` keyword:
function add(a, b)
return a + b
end
Or the short form notation:
add(a, b) = a + b
The use of the `return` keyword is exactly the same as in other languages, but is often
optional. When it's not used, the last expression in the function body will be returned
by default:
function compare(a, b)
a == b && return "equal to"
a < b ? "less than" : "greater than"
end
"""
kw"function"
"""
`return` can be used function bodies to exit early and return a given value, e.g.
function compare(a, b)
a == b && return "equal to"
a < b ? "less than" : "greater than"
end
In general you can place a `return` statement anywhere within a function body, including
within deeply nested loops or conditionals, but be careful with `do` blocks. For
example:
function test1(xs)
for x in xs
iseven(x) && return 2x
end
end
function test2(xs)
map(xs) do x
iseven(x) && return 2x
x
end
end
In the first example, the return breaks out of its enclosing function as soon as it hits
an even number, so `test1([5,6,7])` returns `12`.
You might expect the second example to behave the same way, but in fact the `return`
there only breaks out of the *inner* function (inside the `do` block) and gives a value
back to `map`. `test2([5,6,7])` then returns `[5,12,7]`.
"""
kw"return"
"""
`if`-`elseif`-`else` performs conditional evaluation, which allows portions of code to
be evaluated or not evaluated depending on the value of a boolean expression. Here is
the anatomy of the `if`-`elseif`-`else` conditional syntax:
if x < y
println("x is less than y")
elseif x > y
println("x is greater than y")
else
println("x is equal to y")
end
If the condition expression `x < y` is true, then the corresponding block is evaluated;
otherwise the condition expression `x > y` is evaluated, and if it is true, the
corresponding block is evaluated; if neither expression is true, the `else` block is
evaluated. The `elseif` and `else` blocks are optional, and as many `elseif` blocks as
desired can be used.
"""
kw"if", kw"elseif", kw"else"
"""
`for` loops repeatedly evaluate the body of the loop by iterating over a sequence of
values. For example:
for i in [1,4,0]
println(i)
end
"""
kw"for"
"""
`while` loops repeatedly evaluate a conditional expression, and continues evaluating the
body of the while loop so long as the expression remains `true`. If the condition
expression is false when the while loop is first reached, the body is never evaluated.
For example:
while i <= 5
println(i)
i += 1
end
"""
kw"while"
"""
`end` marks the conclusion of a block of expressions. In the example below, `end` marks
the conclusion of a `function`.
function foo()
println("hello, world")
end
`end` marks the conclusion of all kinds of expression blocks: `module`, `type`, `begin`,
`let`, `for`, etc.
In addition, `end` may be used when indexing into an array to represent the last index
of each dimension:
x[1:end, 2:end-1]
"""
kw"end"
"""
A `try/catch` statement allows for `Exception`s to be tested for. For example, a
customized square root function can be written to automatically call either the real or
complex square root method on demand using `Exception`s:
f(x) = try
sqrt(x)
catch
sqrt(complex(x, 0))
end
`try/catch` statements also allow the `Exception` to be saved in a variable, e.g. `catch y`.
The `catch` clause is not strictly necessary; when omitted, the default return value is
`nothing`. The power of the `try/catch` construct lies in the ability to unwind a deeply
nested computation immediately to a much higher level in the stack of calling functions.
"""
kw"try", kw"catch"
"""
`finally` provides a way to run some code when a given block of code exits, regardless
of how it exits. For example, here is how we can guarantee that an opened file is
closed:
f = open("file")
try
operate_on_file(f)
finally
close(f)
end
When control leaves the `try` block (for example due to a `return`, or just finishing
normally), `close(f)` will be executed. If the `try` block exits due to an exception,
the exception will continue propagating. A `catch` block may be combined with `try` and
`finally` as well. In this case the `finally` block will run after `catch` has handled
the error.
"""
kw"finally"
"""
`break` breaks out of a loop immediately. For example
i = 0
while true
i += 1
i > 10 && break
println(i)
end
prints the numbers 1 to 10.
"""
kw"break"
"""
`continue` skips the rest of the current loop, then carries on looping. For example
for i = 1:10
iseven(i) && continue
println(i)
end
prints the numbers 1, 3, 5..., skipping the even numbers.
"""
kw"continue"
"""
The `do` keyword creates an anonymous function. For example
map(1:10) do x
2x
end
is equivalent to `map(x->2x, 1:10)`.
Use multiple arguments like so:
map(1:10, 11:20) do x, y
x + y
end
"""
kw"do"
"""
The "splat" operator, `...`, represents a sequence of arguments. For example
add(xs...) = reduce(+, xs)
can take any number of arguments:
add(1, 2, 3, 4, 5)
`...` can also be used to apply a function to a sequence of arguments like so:
add([1, 2, 3]...) # 6
add(7, 1:100..., 1000:1100...) # 111107
"""
kw"..."
"""
`;` has a similar role in Julia as in many C-like languages, and is used to delimit the
end of the previous statement. `;` is not necessary after new lines, but can be used to
separate statements on a single line or to join statements into a single expression:
function foo()
println("Hello, "); println("World!")
return true
end
foo() = (println("Hello, World!"); true)
`;` is also used to suppress output in the REPL and similar interfaces.
"""
kw";"
"""
x && y
Short-circuiting boolean AND.
"""
kw"&&"
"""
x || y
Short-circuiting boolean OR.
"""
kw"||"
"""
ccall((symbol, library) or function_pointer, ReturnType, (ArgumentType1, ...), ArgumentValue1, ...)
Call function in C-exported shared library, specified by `(function name, library)`
tuple, where each component is a string or symbol.
Note that the argument type tuple must be a literal tuple, and not a tuple-valued
variable or expression. Alternatively, `ccall` may also be used to call a function
pointer, such as one returned by `dlsym`.
Each `ArgumentValue` to the `ccall` will be converted to the corresponding
`ArgumentType`, by automatic insertion of calls to `unsafe_convert(ArgumentType,
cconvert(ArgumentType, ArgumentValue))`. (See also the documentation for each of these
functions for further details.) In most cases, this simply results in a call to
`convert(ArgumentType, ArgumentValue)`.
"""
kw"ccall"
"""
llvmcall(IR::String, ReturnType, (ArgumentType1, ...), ArgumentValue1, ...)
llvmcall((declarations::String, IR::String), ReturnType, (ArgumentType1, ...), ArgumentValue1, ...)
Call LLVM IR string in the first argument. Similar to an LLVM function `define` block,
arguments are available as consecutive unnamed SSA variables (%0, %1, etc.).
The optional declarations string contains external functions declarations that are
necessary for llvm to compile the IR string. Multiple declarations can be passed in by
separating them with line breaks.
Note that the argument type tuple must be a literal tuple, and not a tuple-valued
variable or expression.
Each `ArgumentValue` to `llvmcall` will be converted to the corresponding
`ArgumentType`, by automatic insertion of calls to `unsafe_convert(ArgumentType,
cconvert(ArgumentType, ArgumentValue))`. (see also the documentation for each of these
functions for further details). In most cases, this simply results in a call to
`convert(ArgumentType, ArgumentValue)`.
See `test/llvmcall.jl` for usage examples.
"""
Core.Intrinsics.llvmcall
"""
`begin...end` denotes a block of code.
begin
println("Hello, ")
println("World!")
end
Usually `begin` will not be necessary, since keywords such as `function` and `let`
implicitly begin blocks of code. See also `;`.
"""
kw"begin"
"""
At their most basic, Julia types are specified as a name and a set of fields.
type Point
x
y
end
Fields can have type restrictions, which may be parametrised:
type Point{X}
x::X
y::Float64
end
Type can also declare an abstract super type via `<:` syntax:
type Point <: AbstractPoint
...
See the manual for more details, such as information on inner constructors.
"""
kw"type"
"""
Introduce a new name for an already expressible type. For example, in `base/boot.jl`,
`UInt` is type aliased to either `UInt64` or `UInt32` as appropriate for the size of
pointers on the system:
if is(Int,Int64)
typealias UInt UInt64
else
typealias UInt UInt32
end
For parametric types, `typealias` can be convenient for providing names in cases where
some parameter choices are fixed. In `base` for example:
typealias Vector{T} Array{T,1}
"""
kw"typealias"
"""
`immutable` acts in the same way as `type`, but declares that the fields of the type may
not be set after construction. See `type` and the manual for more information.
"""
kw"immutable"
"""
@__LINE__ -> Int
`@__LINE__` expands to the line number of the call-site.
"""
kw"@__LINE__"
"""
nothing
The singleton instance of type `Void`, used by convention when there is no value to return
(as in a C `void` function). Can be converted to an empty `Nullable` value.
"""
nothing
"""
ANY
Equivalent to `Any` for dispatch purposes, but signals the compiler to skip code
generation specialization for that field.
"""
ANY
end