[Issue 5007] @outer() attribute

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=5007

--- Comment #8 from bearophile_hugs at eml.cc ---
Walter Bright has commented on @outer():

> I suggest using 'pure' and passing the globals you actually 
> need as ref parameters.

This is what I usually do in D, but it has some small disadvantages:
- It increases the number of function arguments (they are 8 bytes 
each), increasing the size of the function, adding some stack 
management instructions. This slows the function a little, if the 
function is performance-critical.
- Sometimes you can't use "pure", for various reasons, like 
Phobos functions not yet pure, I/O action in your function, or 
other causes.
- If your pure function foo receives two global argument as out 
and ref, the function bar that calls it needs to access to global 
variables or it too needs those two ref/out arguments. The 
current design of @outer() is not transitive, so only foo needs 
to state what global variables are in/out/inout.
- @outer is more DRY, because you don't need to specify the type 
of the global variable received by ref, you just need to know its 
name.
- With @outer you can tighten some old code, without changing the 
signature of a function. If you have an old D module (or a C 
function converted to C) you often can't (or you don't want) to 
change the function signature to add the global arguments passed 
by ref. With @outer() the function signature doesn't change, so 
you can improve your legacy code. It allows a simpler refactoring 
of code.

More notes:
- SPARK language has added a feature similar to @outer, but more 
verbose. See comment 5.
- @outer() is optional and it's fiddly because not it's not meant 
for small D script-like programs, it's meant as a help for 
medium-integrity D programs (where you may think about using Ada 
language instead).

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