Push ifs up and fors down: The idiom, its algebra, and its limits (debasishg.github.io)

59 pointsby speckx3 hours ago20 comments

socializer 2 hours ago

I am continually impressed by the ability of LLMs to take trivial ideas and turn them into lengthy and obtuse blog posts with unnecessary analogies.

bioneuralnet 2 hours ago

Yet another encroachment on traditionally human activity.

moritzwarhier 2 hours ago

I am the

  Option<Walrus>

msdz 8 minutes ago

I know we’re not supposed to comment just for that, but this might be my single favorite joke comment I’ve ever read here. Good job.

swiftcoder 2 hours ago

Honestly, this just looks like one of those lingo-heavy-but-surface-level blog posts that used to make functional programming spaces so insufferable to everyone on the outside

mahboi an hour ago

These things are so divorced from the reality of programming, even when they involve actual code instead of fancy lingo. Like in Scala, not a pure functional language, tutorials used to find the most convoluted higher-order functional way to do simple things.

ninalanyon an hour ago

I've done this for years. Not every time of course but where it makes the code easier to understand and maintain.

Speed was almost never the reason.

alterom an hour ago

I take it you never rewrote a Matlab for loop as a vector/matrix op for insane speedups then :)

wallstop an hour ago

What is missing here is any benchmarks backing up this argument for code structure.

Of note, as of C#9 (and maybe prior), the dotnet runtime does this automatically whenever it is deemed safe. https://devblogs.microsoft.com/dotnet/performance-improvemen...

The same technique is applied as an optimization, when deemed safe, in all current gen c compilers (gcc, llvm, etc).

I'm very confused why neither measurements nor references to when this is done automatically in most modern languages is included in the article.

cogman10 an hour ago

At least in JVM land, it's pretty easy to thwart that optimization. Particularly if the condition is on a mutable yet unchanged in the loop value.

For example:

    var map = new HashMap<String, String>();
    map.put("foo", "bar");
    for (var i : items) {
      if ("bar".equals(map.get("foo")) {
        doStuff(i);
      }
    }

Even though `map` isn't mutated, it's hard enough for the JVM to detect and the underlying `get` functions are complex enough that it'll run the `get("foo")` every time, which can be quiet expensive.

throwawayffffas 11 minutes ago

Just the branch predictor gains are probably worth it.

dieselgate 32 minutes ago

Didn’t see it mentioned in the article but isn’t leading with if-statement called a “guard clause”. I like that pattern but it’s just general best practice I thought.

woadwarrior01 28 minutes ago

Swift explicitly has a guard statement for this. Rust's let .. else { ... } is also very similar.

https://docs.swift.org/latest/documentation/the-swift-progra...

mahboi 9 minutes ago

Guard clauses are things that return early for trivial or problematic cases. Like https://en.wikipedia.org/wiki/Guard_(computer_science)#Flatt...

They're one of those good practices that look like bad practice to everyone who just got a CS degree. Seems ex-students are unsettled by asymmetry or want to minimize the number of return statements.

aappleby an hour ago

I have always phrased this as "Never do one of something".

OutOfHere an hour ago

I like it, but to do fizzbuzz in this way, you'd have to separate what's inside the loop into a reused function.

pdpi 31 minutes ago

I think it's sort of obvious that the limit to this general rule is when data dependencies between fors and ifs forbid you from pushing things further up/down.

taolson 4 minutes ago

Or just use lazy list operations with a single if test at the end:

https://github.com/taolson/Admiran/blob/main/examples/fizzBu...

alterom an hour ago

TL;DR in one sentence:

"the loop runs without a branch, and is a candidate for vectorization".

That's it, that's the article. This matters a lot in huge-scale / scientific computing / HPF, where if you can express something as an operation on vectors on matrices, you win big (those ops parallelize well, can be run on GPUs, clusters, what have you).