I can't offer an answer specific to Haskell, but I know that OCaml does some things to make the compilation fast: interface files, no circular dependencies, no forward declarations, not a lot of optimizations. From what I understand, those tradeoffs come from languages designed by Niklaus Wirth, where efficiency of compilation was important.

In general, I feel like every language that wasn't made to compile fast like Go, OCaml, Pascal and derivatives is going to be called "slow to compile". There's Java and C# that are kind of a middle-ground, since they emit bytecode for a JIT compiler. So my answer to "why Haskell compilation times are so long compared to some other languages?" would be "because compilation times don't take priority over some other points for Haskell and its users".

Haskell compiles so slow because it needs way more optimizations then Ocaml before it performs good. The primary reason for that is its lazy evaluation semantics. Prior to GHC many researchers believed it to be impossible to execute lazy languages with comparable speed than imperative ones. Haskell and GHC were primarily developed as research tools. I.e. they valued the efficient exploration of new ideas in programming languages and implementations higher than the resulting language and implementation, at least in the beginning. And I would say Haskell was extremely successful considering those priorities.

That does not mean, that the implementation hasn't been moving towards industry-readiness for a long time.

If you're compiling a project for the first time it's probably pulling in a ton of libraries and compiling them all from scratch. But let's be real here - we're talking maybe 3-5 minutes for the first build of a huge Haskell codebase, one or two orders of magnitude faster on second build.

It seems comparable to what I've noticed with Rust or C, and pretty fast compared to some medium/large C++ codebases I've built.

I'm not really sure what workflows people are using where this is a problem, so this may be less of a pain point for me than it is for you. Personally I build Haskell projects once to pull in the dependencies, then use ghcid to get ~instantaneous typechecking on changes I make to the codebase.

From my experience with Haskell and Scala, derivations and not dependencies is what’s driving compile time. You can probably hide a crypto miner behind all that compiler magic. In Scala I usually move code that invokes a lot of derivations into sub projects, not a clue on how to do that in Haskell

I wish it took 3 - 5 minutes, I’ve built it projects that are far longer than that. I know this isn’t just me though, because the GHC build documentation tells you to go read stuff for a while as it builds. Longest I can remember is 2 hours.

> moving closer to the heat death of the universe

So… you're saying your laptop is super cool? Because the heat death of the universe is when thermodynamic energy is equally distributed everywhere, which, given the large space of the universe, means really cold.

Heh. Though while we're playing Pedantics, it's worth pointing out that the defining characteristic of Heat Death is maximum entropy, not uniform temperature. That said, it's entirely possible that the whole system of Energy Company + OP's computer actually does get cooler on average when compiling Haskell.

Here's a fun game. Have you ever played Follow the Energy? For example, depressing keys on your keyboard takes energy. Where does that energy come from? Well, the work performed by your fingers of course! But where does that energy come from? Well, the muscles in your fingers, of course! But where does that come from? Well, your food! But what about that? Well, your food's food! That? Eventually plants. That? The sun! That? Nuclear fusion. That? Gravitational potential! That? Etc...

But here's the funny thing. In this image, it kind of seems like energy gets "used up". The sun provides energy to the earth via solar radiation; the plants consume this energy; animals eat the plants, obtaining their energy; etc. However, energy is conserved. What gives?

Better yet, if the earth were not radiating just as much energy as it received, it should be heating up. However, the earth-atmosphere system is mostly constant temperature! This implies that the total energy flux is zero. If X Watts (Energy/Time) is coming in, then the earth actually radiates out X Watts as well.

So... if the total energy flux is zero, then how does your keyboard key actually get pressed? What gives?

The key is entropy. X Watts of solar radiation impinge upon the earth, but these photons are "hotter" (i.e. higher frequency) on average than those that radiate out. The balance is in numbers. You need more cooler photons to balance the energy of the hotter ones. E=hf; energy (E) is proportional (constant h) to frequency (f), after all.

This means that while energy is conserved, the flow of that energy increases the entropy of the system. In a very real sense, typing on your keyboard happens because of the waste heat generated in the process.

All driving us closer to Heat Death...

> The sun! That? Nuclear fusion. That? Gravitational potential! That? Etc...

Minor pedantry on top of pedantry. The gravitational potential energy of a collapsing protostar gets you past the activation barrier for nuclear fusion, but isn't itself the source of energy beyond that.

Think of it like lighting campfire using friction. The friction heats up the kindling, but that investment allows you to access the potential chemical energy of the wood. The gravitational potential energy is converted into heat, and that investment allows you to access the potential nuclear energy of the unfused hydrogen.

Thanks! That's one place in the chain that I paused for quite a while, deliberating what to write down. Overall, the little "causal chain" I wrote down is littered with little (but useful!) fibs, and the one you point out is probably the most egregious.

Nice analogy, btw.

Thanks for elaborating on this. I guess the confusion also stems from the word "heat" which we usually associate with something that's warmer than some human day-to-day reference. Heat in the physical sense though just refers to the kinetic energy of a number of particles, which can be harvested (per the post above) provided there are differences in kinetic energy throughout space.

It's also interesting to play the "Follow the Energy" game to it's logical conclusion, namely that nearly all of the energy in the Universe (including that which you expend typing on your keyboard) originates from the gravitational potential created in the Big Bang (whatever that is, by the way). This begs the question; how was the entropy of the early Universe to low, that it could increase by such an enormous amount, to produce intelligent beings such as ourselves, typing things on a keyboard while we should be working?

It's really one of the most fundamental questions in cosmology, and one of the (many) reasons why I love physics.

Actually, the heat death is when all potential energy in the universe has been converted to heat. So converting an excess of stored chemical energy in their laptop battery to heat by compiling a load of Haskell would be a fine way of increasing the entropy of the Universe. Thus moving ever so slightly closer to the inevitable heat death.