Travis, IMHO most Haskell features give a great benefit with a very small cost, but laziness is the exception: It gives a great benefit, but at a fairly significant cost. That cost comes in the form of a learning curve for dealing with space behaviour. I would add that even though Haskell has some faults, it is overall an amazingly practical language - possibly the best around at this point in time. Out of all the gripes that people rightly make about certain details of Haskell, the only one that causes any real grief is space behaviour. If we can ease the pain of space behaviour, then Haskell can be all pleasure. I hope I can help with that! The first thing to understand is that "a `seq` b" means "when you evaluate me, evaluate a and b, and return b". It doesn't actually force evaluation, it just ties the valuation of one thing to the evaluation of another. It's equivalent to just "b" except that it also implies the evaluation of a. The next thing to understand is that "a `seq` b" only causes evaluation of the outermost constructor of a. For some "strict" types the outermost constructor is the whole value, so `seq` causes the whole expression to be evaluated, e.g. Int Bool newtype Metres = Metres Float data Point2 = Point2 !Int !Int Even if it's a simple type, any unevaluated expression can potentially consume large amounts of memory, because the runtime system can't garbage collect whatever data structures are needed to evaluate it. Then you can get space leaks. With these strict types, life is easy - you can just use `seq` or foldl' or such like, and then you can guarantee that these references are no longer hanging around. But a lot of types are lazy. Here are some examples. Their contained values aren't evaluated by a simple `seq`: Maybe a data Metres = Metres Float (a,b) [a] Data.Set.Set a Another point is that the "a" in "a `seq` b" needs to be a let binding so it's the same "a" as you use in other places, e.g. let total' = total + value n' = n + 1 in total' `seq` n' `seq` (total, n) Because of the two `seq`s, the value of this expression is now strict, so that using `seq` on it will force the whole thing, even though it is of type (a, b) which is normally a lazy type. So this means it's possible to make a normally lazy value - or specific parts of it - strict by making sure that each publicly exposed function that constructs the value forces evaluation using appropriate `seq`s. For example, the keys in a Data.Map structure are treated strictly, and the values lazily. This strictness is guaranteed by each function that manipulates Data.Map being written so as to give this guarantee, and then a promise is made in the API documentation. It's important to document space behaviour, since the caller does need to know it. So that's how to control evaluation directly. The Control.Parallel.Strategies module from 'parallel' contains helpers for managing evaluation (useful in non-parallel software too). I think ultimately if you want to put Haskell to practice use, you do need to be aware of Haskell's evaluation behaviour. Until you really understand it, you will be occasionally plagued by space leaks. In my experience, space leaks have been tractable with a combination of space profiling (which is unavoidable sometimes for big programs) and good understanding, which has taken a bit of effort to acquire. Steve Travis Erdman wrote:
The driver of my algorithm looks like this:
foldl' processfn nullarray (take arg infinitelist)
where processfn takes an array and the next set of inputs, processes, and delivers a new updated array (using the Data.Vector library).
Apparently, I have a space leak ... the "maximum residency" varies linearly with the size of "arg" supplied, garbage collection consumes ~75% of CPU time, and, if the arg is too big, the whole thing crashes with an out of memory error. The algorithm should operate in constant space.
As you can see, I'm using a strict left-fold and also have made the accumulating array strict in the processfn definition with a bang pattern. So, I'm sort of at a loss as to how to resolve this.
The help provided on this list has been outstanding, thanks to all of you; hope you have something left in the tank for this one!
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