Nim/doc/effects.txt

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               Side effects in Nim
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Note: Side effects are implicit produced values! Maybe they should be
explicit like in Haskell?


The idea is that side effects and partial evaluation belong together:
Iff a proc is side effect free and all its argument are evaluable at
compile time, it can be evaluated by the compiler. However, really
difficult is the ``newString`` proc: If it is simply wrapped, it
should not be evaluated at compile time! On other occasions it can
and should be evaluted:

.. code-block:: nim
  proc toUpper(s: string): string =
    result = newString(len(s))
    for i in 0..len(s) - 1:
      result[i] = toUpper(s[i])

No, it really can always be evaluated. The code generator should transform
``s = "\0\0\0..."`` back into ``s = newString(...)``.


``new`` cannot be evaluated at compile time either.


Raise statement
===============

It is impractical to consider ``raise`` as a statement with side effects.


Solution
========

Being side effect free does not suffice for compile time evaluation. However,
the evaluator can attempt to evaluate at compile time.