[fpc-devel] Pure function Wiki page

[J. Gareth Moreton]

 The problem with determining if a function is pure or not is not just
limited to looking for references to external memory and the like, but also
has to take into account that the function might contain an infinite loop,
for example, and this is difficult because there is no general-purpose
algorithm for finding out if a given program (or a group of functions) will
finish or continue running forever.  See "Turing's halting problem":
https://en.wikipedia.org/wiki/Halting_problem
 As mentioned before, the other reason is semantics.  It might potentially
confuse the programmer if "const x = FunctionCall(42);" is seemingly
allowed for some arbitrary functions, but not others.  By clearly defining
valid functions as pure, it makes it clear to the programmer what can be
specified, and also aids in debugging because the compiler will throw a
warning if the function is question is actually impure, for example (and
then the actual error for trying to assign a function result to a
constant).

 Gareth

 On Mon 09/07/18 00:00 , Dmitry Boyarintsev skalogryz.lists at gmail.com sent:
 On Sun, Jul 8, 2018 at 6:47 PM, Thorsten Engler  wrote:

 You are thinking about something like:

 

const

  x = FunctionCall(42);

  

In which case, yes, the compiler could possibly see that as in implicit
“check if that function is pure”.

 

But “constant expressions” can also be something like: 

 

If FunctionCall(42) > 0 then 

 

 In which case you don’t want the compiler to have to test every single
expression in your program to see if it is composed (all the way down) of
pure functions.
 Maybe a different approach should be taken?Determine how much performance
impact is made to determine the purity of a function. and then consider
adding a new directive and a keyword.
 thanks,
 Dmitry _______________________________________________
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