Hacker News Reader

Comment by nepeckman | Hacker News Reader

> func is now an alias for proc {.noSideEffect.}

So excited for this addition! I like to program Nim in a functional lite style, and this is some nice sugar to write functional procedures. Its also nice that the language reserved the function keyword for procedures that are _actually_ functions.

View on HN

> So excited for this addition!

I've been using it for a while in the devel version (Nim v0.18.1), and it is great — I like it very much and I completely understand your excitement!

Before I didn't bother with {.noSideEffect.} pragma, and now with `func` here — my code is much more clear (at least to me ;)), and I can immediately tell which of my procs are 'pure'.

What does it do?

It does some static enforcement of "no side effects[1]".

[1]https://en.m.wikipedia.org/wiki/Side_effect_(computer_scienc...

How? Even Haskell, who makes a big deal out of separating pure vs impure code, can't do it perfectly.

I don't think they claim they do it perfectly. You can find bugs related to that, false negatives and positives.

https://nim-lang.org/docs/manual.html#pragmas-nosideeffect-p...

It looks really impressive.

"Haskell is the world’s best imperative programming language"

I want to use the func keword more. I think many of my procs can be funcs instead. In my programs i like having a simple imperative middle, but fuctional no side effects edges.

can someone expound on what this does exactly and how it works? Will the compiler try to statically verify that it is actually pure and error if it can't? Does it unlock optimizations or other features?

Yes and yes. From the manual:

> If none of its parameters have the type var T or ref T or ptr T this means no locations are modified. It is a static error to mark a proc/iterator to have no side effect if the compiler cannot verify this.

I'm not sure how many compiler optimizations are possible, but I do know that procs marked with no side effect are automatically considered thread safe.

What if your func iamsafe calls the proc imodifyglobals? More generally, can a "func" call a proc and remain a func?

I'm not really a Nim expert, but I did a quick test on the scenario you laid out with these results: iamsafe could not call imodifyglobals, but could call a proc with no side effects. I'd guess that a func can call out to any proc that the compiler knows is side effect free, with the compiler erring on the side of caution.

Yep, AFAIK the compiler will do its best to infer the effects and most of the time it does a pretty good job.

Nope, it can only call other side-effect free things. Note however if you call a procedure from a function the function will try to see if the proc is actually side effect free, just not marked as such. So you can still call a proc, but only if it could've been a func.

Reply on news.ycombinator.com