.. _man-introduction:

**************
 Introduction  
**************

Scientific computing has traditionally required the highest performance,
yet domain experts have largely moved to slower dynamic languages for
daily work. We believe there are many good reasons to prefer dynamic
languages for these applications, and we do not expect their use to
diminish. Fortunately, modern language design and compiler techniques
make it possible to mostly eliminate the performance trade-off and
provide a single environment productive enough for prototyping and
efficient enough for deploying performance-intensive applications. The
Julia programming language fills this role: it is a flexible dynamic
language, appropriate for scientific and numerical computing, with
performance comparable to traditional statically-typed languages.

Because Julia's compiler is different from the interpreters used
for languages like Python or R, you may find that Julia's performance
is unintuitive at first. If you find that something is slow, we highly
recommend reading through the
`Performance Tips <http://docs.julialang.org/en/latest/manual/performance-tips>`_
section before trying anything else. Once you understand how Julia 
works, it's easy to write code that's nearly as fast as C.

Julia features optional typing, multiple dispatch, and good
performance, achieved using type inference and `just-in-time (JIT)
compilation <http://en.wikipedia.org/wiki/Just-in-time_compilation>`_,
implemented using `LLVM
<http://en.wikipedia.org/wiki/Low_Level_Virtual_Machine>`_. It is
multi-paradigm, combining features of imperative, functional, and
object-oriented programming. Julia provides ease and expressiveness
for high-level numerical computing, in the same way as languages such
as R, MATLAB, and Python, but also supports general programming. To
achieve this, Julia builds upon the lineage of
mathematical programming languages, but also borrows much from popular
dynamic languages, including `Lisp
<http://en.wikipedia.org/wiki/Lisp_(programming_language)>`_, `Perl
<http://en.wikipedia.org/wiki/Perl_(programming_language)>`_, `Python
<http://en.wikipedia.org/wiki/Python_(programming_language)>`_, `Lua
<http://en.wikipedia.org/wiki/Lua_(programming_language)>`_, and `Ruby
<http://en.wikipedia.org/wiki/Ruby_(programming_language)>`_.

The most significant departures of Julia from typical dynamic languages
are:

-  The core language imposes very little; the standard library is
   written in Julia itself, including primitive operations like integer
   arithmetic
-  A rich language of types for constructing and describing objects,
   that can also optionally be used to make type declarations
-  The ability to define function behavior across many combinations of
   argument types via `multiple
   dispatch <http://en.wikipedia.org/wiki/Multiple_dispatch>`_
-  Automatic generation of efficient, specialized code for different
   argument types
-  Good performance, approaching that of statically-compiled languages
   like C

Although one sometimes speaks of dynamic languages as being "typeless",
they are definitely not: every object, whether primitive or
user-defined, has a type. The lack of type declarations in most dynamic
languages, however, means that one cannot instruct the compiler about
the types of values, and often cannot explicitly talk about types at
all. In static languages, on the other hand, while one can — and usually
must — annotate types for the compiler, types exist only at compile time
and cannot be manipulated or expressed at run time. In Julia, types are
themselves run-time objects, and can also be used to convey information
to the compiler.

While the casual programmer need not explicitly use types or multiple
dispatch, they are the core unifying features of Julia: functions are
defined on different combinations of argument types, and applied by
dispatching to the most specific matching definition. This model is a
good fit for mathematical programming, where it is unnatural for the
first argument to "own" an operation as in traditional object-oriented
dispatch. Operators are just functions with special notation — to extend
addition to new user-defined data types, you define new methods for the
``+`` function. Existing code then seamlessly applies to the new data
types.

Partly because of run-time type inference (augmented by optional type
annotations), and partly because of a strong focus on performance from
the inception of the project, Julia's computational efficiency exceeds
that of other dynamic languages, and even rivals that of
statically-compiled languages. For large scale numerical problems, speed
always has been, continues to be, and probably always will be crucial:
the amount of data being processed has easily kept pace with Moore's Law
over the past decades.

Julia aims to create an unprecedented combination of ease-of-use, power,
and efficiency in a single language. In addition to the above, some
advantages of Julia over comparable systems include:

-  Free and open source (`MIT
   licensed <https://github.com/JuliaLang/julia/blob/master/LICENSE.md>`_)
-  User-defined types are as fast and compact as built-ins
-  No need to vectorize code for performance; devectorized code is fast
-  Designed for parallelism and distributed computation
-  Lightweight "green" threading
   (`coroutines <http://en.wikipedia.org/wiki/Coroutine>`_)
-  Unobtrusive yet powerful type system
-  Elegant and extensible conversions and promotions for numeric and
   other types
-  Efficient support for
   `Unicode <http://en.wikipedia.org/wiki/Unicode>`_, including but not
   limited to `UTF-8 <http://en.wikipedia.org/wiki/UTF-8>`_
-  Call C functions directly (no wrappers or special APIs needed)
-  Powerful shell-like capabilities for managing other processes
-  Lisp-like macros and other metaprogramming facilities

