Syntax
======

This section lists Nim's standard syntax. How the parser handles
the indentation is already described in the `Lexical Analysis`_ section.

Nim allows user-definable operators.
Binary operators have 11 different levels of precedence.



Associativity
-------------

Binary operators whose first character is ``^`` are right-associative, all
other binary operators are left-associative.

.. code-block:: nim
  proc `^/`(x, y: float): float =
    # a right-associative division operator
    result = x / y
  echo 12 ^/ 4 ^/ 8 # 24.0 (4 / 8 = 0.5, then
                           12 / 0.5 = 24.0)
  echo 12  / 4  / 8 # 0.375 (12 / 4 = 3.0, then
                              3 / 8 = 0.375)

Precedence
----------

Unary operators always bind stronger than any binary
operator: ``$a + b`` is ``($a) + b`` and not ``$(a + b)``.

If an unary operator's first character is ``@`` it is a `sigil-like`:idx:
operator which binds stronger than a ``primarySuffix``: ``@x.abc`` is parsed
as ``(@x).abc`` whereas ``$x.abc`` is parsed as ``$(x.abc)``.


For binary operators that are not keywords the precedence is determined by the
following rules:

Operators ending in either ``->``, ``~>`` or ``=>`` are called
`arrow like`:idx:, and have the lowest precedence of all operators.

If the operator ends with ``=`` and its first character is none of
``<``, ``>``, ``!``, ``=``, ``~``, ``?``, it is an *assignment operator* which
has the second lowest precedence.

Otherwise precedence is determined by the first character.

================  ===============================================  ==================  ===============
Precedence level    Operators                                      First character     Terminal symbol
================  ===============================================  ==================  ===============
 10 (highest)                                                      ``$  ^``            OP10
  9               ``*    /    div   mod   shl  shr  %``            ``*  %  \  /``      OP9
  8               ``+    -``                                       ``+  -  ~  |``      OP8
  7               ``&``                                            ``&``               OP7
  6               ``..``                                           ``.``               OP6
  5               ``==  <= < >= > !=  in notin is isnot not of``   ``=  <  >  !``      OP5
  4               ``and``                                                              OP4
  3               ``or xor``                                                           OP3
  2                                                                ``@  :  ?``         OP2
  1               *assignment operator* (like ``+=``, ``*=``)                          OP1
  0 (lowest)      *arrow like operator* (like ``->``, ``=>``)                          OP0
================  ===============================================  ==================  ===============


Whether an operator is used a prefix operator is also affected by preceeding whitespace (this parsing change was introduced with version 0.13.0):

.. code-block:: nim
  echo $foo
  # is parsed as
  echo($foo)


Strong spaces
-------------

The number of spaces preceding a non-keyword operator affects precedence
if the experimental parser directive ``#?strongSpaces`` is used. Indentation
is not used to determine the number of spaces. If 2 or more operators have the
same number of preceding spaces the precedence table applies, so ``1 + 3 * 4``
is still parsed as ``1 + (3 * 4)``, but ``1+3 * 4`` is parsed as ``(1+3) * 4``:

.. code-block:: nim
  #? strongSpaces
  if foo+4 * 4 == 8  and  b&c | 9  ++
      bar:
    echo ""
  # is parsed as
  if ((foo+4)*4 == 8) and (((b&c) | 9) ++ bar): echo ""


Furthermore whether an operator is used a prefix operator is affected by the
number of spaces:

.. code-block:: nim
  #? strongSpaces
  echo $foo
  # is parsed as
  echo($foo)

This also affects whether ``[]``, ``{}``, ``()`` are parsed as constructors
or as accessors:

.. code-block:: nim
  #? strongSpaces
  echo (1,2)
  # is parsed as
  echo((1,2))

Only 0, 1, 2, 4 or 8 spaces are allowed to specify precedence and it is
enforced that infix operators have the same amount of spaces before and after
them. This rules does not apply when a newline follows after the operator,
then only the preceding spaces are considered.


Grammar
-------

The grammar's start symbol is ``module``.

.. include:: grammar.txt
   :literal:

