=========================================================
   Term rewriting macros for Nimrod
=========================================================

:Author: Andreas Rumpf

Term rewriting macros are macros or templates that have not only a *name* but
also a *pattern* that is searched for after the semantic checking phase of
the compiler: This means they provide an easy way to enhance the compilation
pipeline with user defined optimizations:

.. code-block:: nimrod
  template optMul{`*`(a, 2)}(a: int): int = a+a
  
  let x = 3
  echo x * 2

The compiler now rewrites ``x * 2`` as ``x + x``. The code inside the
curlies is the pattern to match against. The operators ``*``,  ``**``,
``|``, ``~`` have a special meaning in patterns if they are written in infix 
notation, so to match verbatim against ``*`` the ordinary function call syntax
needs to be used.


Unfortunately optimizations are hard to get right and even the tiny example
is **wrong**: 

.. code-block:: nimrod
  template optMul{`*`(a, 2)}(a: int): int = a+a
  
  proc f(): int =
    echo "side effect!"
    result = 55
  
  echo f() * 2

We cannot duplicate 'a' if it denotes an expression that has a side effect!
Fortunately Nimrod supports side effect analysis:

.. code-block:: nimrod
  template optMul{`*`(a, 2)}(a: int{noSideEffect}): int = a+a
  
  proc f(): int =
    echo "side effect!"
    result = 55
  
  echo f() * 2 # not optimized ;-)

So what about ``2 * a``? We should tell the compiler ``*`` is commutative. We
cannot really do that however as the following code only swaps arguments
blindly:

.. code-block:: nimrod
  template mulIsCommutative{`*`(a, b)}(a, b: int): int = b*a
  
What optimizers really need to do is a *canonicalization*:

.. code-block:: nimrod
  template canonMul{`*`(a, b)}(a: int{lit}, b: int): int = b*a

The ``int{lit}`` parameter pattern matches against an expression of 
type ``int``, but only if it's a literal.



Parameter constraints
=====================

The parameter constraint expression can use the operators ``|`` (or), 
``&`` (and) and ``~`` (not) and the following predicates:

===================      =====================================================
Predicate                Meaning
===================      =====================================================
``atom``                 The matching node has no children.
``lit``                  The matching node is a literal like "abc", 12.
``sym``                  The matching node must be a symbol (a bound 
                         identifier).
``ident``                The matching node must be an identifier (an unbound
                         identifier).
``call``                 The matching AST must be a call/apply expression.
``lvalue``               The matching AST must be an lvalue.
``sideeffect``           The matching AST must have a side effect.
``nosideeffect``         The matching AST must have no side effect.
``param``                A symbol which is a parameter.
``genericparam``         A symbol which is a generic parameter.
``module``               A symbol which is a module.
``type``                 A symbol which is a type.
``var``                  A symbol which is a variable.
``let``                  A symbol which is a ``let`` variable.
``const``                A symbol which is a constant.
``result``               The special ``result`` variable.
``proc``                 A symbol which is a proc.
``method``               A symbol which is a method.
``iterator``             A symbol which is an iterator.
``converter``            A symbol which is a converter.
``macro``                A symbol which is a macro.
``template``             A symbol which is a template.
``field``                A symbol which is a field in a tuple or an object.
``enumfield``            A symbol which is a field in an enumeration.
``forvar``               A for loop variable.
``label``                A label (used in ``block`` statements).
``nk*``                  The matching AST must have the specified kind. 
                         (Example: ``nkIfStmt`` denotes an ``if`` statement.)
``alias``                States that the marked parameter needs to alias 
                         with *some* other parameter.
``noalias``              States that *every* other parameter must not alias
                         with the marked parameter.
===================      =====================================================

The ``alias`` and ``noalias`` predicates refer not only to the matching AST,
but also to every other bound parameter; syntactially they need to occur after
the ordinary AST predicates:

.. code-block:: nimrod
  template ex{a = b + c}(a: int{noalias}, b, c: int) =
    # this transformation is only valid if 'b' and 'c' do not alias 'a':
    a = b
    inc a, b


Pattern operators
=================

The operators ``*``,  ``**``, ``|``, ``~`` have a special meaning in patterns
if they are written in infix notation.


The ``|`` operator
------------------

The ``|`` operator if used as infix operator creates an ordered choice:

.. code-block:: nimrod
  template t{0|1}(): expr = 3
  let a = 1
  # outputs 3:
  echo a

The matching is performed after the compiler performed some optimizations like
constant folding, so the following does not work:

.. code-block:: nimrod
  template t{0|1}(): expr = 3
  # outputs 1:
  echo 1

The reason is that the compiler already transformed the 1 into "1" for
the ``echo`` statement. However, a term rewriting macro should not change the
semantics anyway. In fact they can be deactived with the ``--patterns:off``
command line option or temporarily with the ``patterns`` pragma. 


The ``{}`` operator
-------------------

A pattern expression can be bound to a pattern parameter via the ``expr{param}``
notation: 

.. code-block:: nimrod
  template t{(0|1|2){x}}(x: expr): expr = x+1
  let a = 1
  # outputs 2:
  echo a


The ``~`` operator
------------------

The ``~`` operator is the **not** operator in patterns:

.. code-block:: nimrod
  template t{x = (~x){y} and (~x){z}}(x, y, z: bool): stmt =
    x = y
    if x: x = z
  
  var
    a = false
    b = true
    c = false
  a = b and c
  echo a


The ``*`` operator
------------------

The ``*`` operator can *flatten* a nested binary expression like ``a & b & c``
to ``&(a, b, c)``: 

.. code-block:: nimrod
  var
    calls = 0
    
  proc `&&`(s: varargs[string]): string =
    result = s[0]
    for i in 1..len(s)-1: result.add s[i]
    inc calls

  template optConc{ `&&` * a }(a: string): expr = &&a

  let space = " "
  echo "my" && (space & "awe" && "some " ) && "concat"

  # check that it's been optimized properly:
  doAssert calls == 1


The second operator of `*` must be a parameter; it is used to gather all the
arguments. The expression ``"my" && (space & "awe" && "some " ) && "concat"``
is passed to ``optConc`` in ``a`` as a special list (of kind ``nkArgList``) 
which is flattened into a call expression; thus the invocation of ``optConc`` 
produces:

.. code-block:: nimrod
   `&&`("my", space & "awe", "some ", "concat")


The ``**`` operator
-------------------

The ``**`` is much like the ``*`` operator, except that it gathers not only
all the arguments, but also the matched operators in reverse polish notation:

.. code-block:: nimrod
  import macros

  type
    TMatrix = object
      dummy: int

  proc `*`(a, b: TMatrix): TMatrix = nil
  proc `+`(a, b: TMatrix): TMatrix = nil
  proc `-`(a, b: TMatrix): TMatrix = nil
  proc `$`(a: TMatrix): string = result = $a.dummy
  proc mat21(): TMatrix =
    result.dummy = 21

  macro optM{ (`+`|`-`|`*`) ** a }(a: TMatrix): expr =
    echo treeRepr(a)
    result = newCall(bindSym"mat21")

  var x, y, z: TMatrix

  echo x + y * z - x 

This passes the expression ``x + y * z - x`` to the ``optM`` macro as
an ``nnkArgList`` node containing::

  Arglist
    Sym "x"
    Sym "y"
    Sym "z"
    Sym "*"
    Sym "+"
    Sym "x"
    Sym "-"

(Which is the reverse polish notation of ``x + y * z - x``.)


Parameters
==========

Parameters in a pattern are type checked in the matching process. If a 
parameter is of the type ``varargs`` it is treated specially and it can match
0 or more arguments in the AST to be matched against:

.. code-block:: nimrod
  template optWrite{
    write(f, x)
    ((write|writeln){w})(f, y)
  }(x, y: varargs[expr], f: TFile, w: expr) =
    w(f, x, y)
  
