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Comment by cicero | Hacker News Reader

I have heard critiques of functional programming, but not that there is a problem with leaky abstractions. Can you provide an example? Does it invalidate the discipline of trying to use pure functions when possible?

I learned to program using BASIC on an Apple II. All of the variables were global, and it wasn't until I got Apple Pascal that I had access to a language that had local variables. I immediately saw the advantage of the discipline of using local variables whenever possible because I had experienced the difficulty of tracking down where in the whole program a global variable might be changed.

It could be argued that local variables are just syntactic sugar over a global memory space, but no one credibly argues today that we should therefore only use global variables. Local variables make it easier to reason about the behavior of a program, and it appears to me that functional programming takes that idea even further.

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Program execution time and memory footprint are great examples. Math does not care about these details, but just because two programs are logically equivalent does not mean they will behave identically.

I am very pro functional programming, but your mental model needs to be accurate to really dig into the details. As such you really need to understand ASM and procedural code or it will eventually bite you.

That's a curious place to draw the line. As we've seen in the past year, assembly isn't good enough. You need to understand your microcode or it will eventually bite you ... and you don't.

But that's all just nit-picking <<1% cases. When's the last time you saw a functional program whose execution time or memory footprint were unacceptable, and required knowledge of assembly language (or lower) to resolve? I can't say I ever have.

The best example I can give is not quite functional code. I have seen a lot of horrific SQL because someone did not really understand that Joins for example had a cost based on some understandable criteria. You need to have a basic mental model of what the database does to write good SQL.

I don't expect everyone to proficient in ASM let alone microcode, but I do expect them to at least understand that they exist. Beyond that I expect any decent CS program to enable someone to build a useful mental model of what's going on. Plenty of subject matter experts write very valuable code without that knowledge, but they tend to hit a very real wall.

Perhaps I'm missing the point here, but my understanding is that functional programmers (and especially Haskell programmers) tend to have a different problem: knowing what code the compiler can optimise well.

From what I've heard about tuning Haskell code for performance, much of it depends on the particulars of the compiler, rather than on the underlying CPU.

I don't like how people equate the algebraic FP style of programming with "math" -- there are plenty of ways to model execution time and memory footprint using math, for example. A functional program is no more "mathematical" than an imperative one.

I mean as a mental model.

You can for example analyze a program assuming an ideal compiler (which I have seen people do), or you can understand the actual compiler used. The second is an implementation detail subject to change, but it’s also more accurate.

I recall one meeting when people said doing comparisons with a list of objects one comparison at a time should be as fast as doing the same thing one object at a time. I pointed out that due to cache issues doing each check an object at a time would be faster. A few people where shocked when the test showed a significant improvement.

Purely functional languages come with the promise that a sufficiently advanced compiler will see through all that monadic functional cruft and run your code as well or even better than it would if written in an imperative language.

As we don’t yet have such an advanced compiler, impure hacks like `par` start seeping through the cracks.

What do you mean impure hacks like `par`? It's a parallelism primitive. You mean `seq`? In either case it's not impure. It's just a primitive that can't be expressed in the language itself. It's still referentially transparent (which is what people mean by pure).

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