What are Haskell's strictness points?

Question

We all know (or should know) that Haskell is lazy by default. Nothing is evaluated until it must be evaluated. So when must something be evaluated? There are points where Haskell must be strict. I call these "strictness points", although this particular term isn't as widespread as I had thought. According to me:

Reduction (or evaluation) in Haskell only occurs at strictness points.

So the question is: what, precisely, are Haskell's strictness points? My intuition says that main, seq / bang patterns, pattern matching, and any IO action performed via main are the primary strictness points, but I don't really know why I know that.

(Also, if they're not called "strictness points", what are they called?)

I imagine a good answer will include some discussion about WHNF and so on. I also imagine it might touch on lambda calculus.

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Edit: additional thoughts about this question.

As I've reflected on this question, I think it would be clearer to add something to the definition of a strictness point. Strictness points can have varying contexts and varying depth (or strictness). Falling back to my definition that "reduction in Haskell only occurs at strictness points", let us add to that definition this clause: "a strictness point is only triggered when its surrounding context is evaluated or reduced."

So, let me try to get you started on the kind of answer I want. main is a strictness point. It is specially designated as the primary strictness point of its context: the program. When the program (main's context) is evaluated, the strictness point of main is activated. Main's depth is maximal: it must be fully evaluated. Main is usually composed of IO actions, which are also strictness points, whose context is main.

Now you try: discuss seq and pattern matching in these terms. Explain the nuances of function application: how is it strict? How is it not? What about deepseq? let and case statements? unsafePerformIO? Debug.Trace? Top-level definitions? Strict data types? Bang patterns? Etc. How many of these items can be described in terms of just seq or pattern matching?

Answer 1

This is not a full answer aiming for karma, but just a piece of the puzzle -- to the extent that this is about semantics, bear in mind that there are multiple evaluation strategies that provide the same semantics. One good example here -- and the project also speaks to how we typically think of Haskell semantics -- was the Eager Haskell project, which radically altered evaluation strategies while maintaining the same semantics: http://csg.csail.mit.edu/pubs/haskell.html

Answer 2

Lazy doesn't mean do nothing. Whenever your program pattern matches a case expression, it evaluates something -- just enough anyway. Otherwise it can't figure out which RHS to use. Don't see any case expressions in your code? Don't worry, the compiler is translating your code to a stripped down form of Haskell where they are hard to avoid using.

For a beginner, a basic rule of thumb is let is lazy, case is less lazy.

Answer 3

The Glasgow Haskell compiler translates your code into a Lambda-calculus-like language called core. In this language, something is going to be evaluated, whenever you pattern match it by a case-statement. Thus if a function is called, the outermost constructor and only it (if there are no forced fields) is going to be evaluated. Anything else is canned in a thunk. (Thunks are introduced by let bindings).

Of course this is not exactly what happens in the real language. The compiler convert Haskell into Core in a very sophisticated way, making as many things as possibly lazy and anything that is always needed lazy. Additionally, there are unboxed values and tuples that are always strict.

If you try to evaluate a function by hand, you can basically think:

• Try to evaluate the outermost constructor of the return.
• If anything else is needed to get the result (but only if it's really needed) is also going to be evaluated. The order doesn't matters.
• In case of IO you have to evaluate the results of all statements from the first to the last in that. This is a bit more complicated, since the IO monad does some tricks to force evaluation in a specific order.

Answer 4

C has the concept of sequence points, which are guarantees for particular operations that one operand will be evaluated before the other. I think that's the closest existing concept, but the essentially equivalent term strictness point (or possibly force point) is more in line with Haskell thinking.

In practice Haskell is not a purely lazy language: for instance pattern matching is usually strict (So trying a pattern match forces evaluation to happen at least far enough to accept or reject the match.

…

Programmers can also use the seq primitive to force an expression to evaluate regardless of whether the result will ever be used.

$! is defined in terms of seq.

—Lazy vs. non-strict.

So your thinking about !/$! and seq is essentially right, but pattern matching is subject to subtler rules. You can always use ~ to force lazy pattern matching, of course. An interesting point from that same article:

The strictness analyzer also looks for cases where sub-expressions are always required by the outer expression, and converts those into eager evaluation. It can do this because the semantics (in terms of "bottom") don't change.

Let's continue down the rabbit hole and look at the docs for optimisations performed by GHC:

Strictness analysis is a process by which GHC attempts to determine, at compile-time, which data definitely will 'always be needed'. GHC can then build code to just calculate such data, rather than the normal (higher overhead) process for storing up the calculation and executing it later.

—GHC Optimisations: Strictness Analysis.

In other words, strict code may be generated anywhere as an optimisation, because creating thunks is unnecessarily expensive when the data will always be needed (and/or may only be used once).

…no more evaluation can be performed on the value; it is said to be in normal form. If we are at any of the intermediate steps so that we've performed at least some evaluation on a value, it is in weak head normal form (WHNF). (There is also a 'head normal form', but it's not used in Haskell.) Fully evaluating something in WHNF reduces it to something in normal form…

—Wikibooks Haskell: Laziness

(A term is in head normal form if there is no beta-redex in head position¹. A redex is a head redex if it is preceded only by lambda abstractors of non-redexes ².) So when you start to force a thunk, you're working in WHNF; when there are no more thunks left to force, you're in normal form. Another interesting point:

…if at some point we needed to, say, print z out to the user, we'd need to fully evaluate it…

Which naturally implies that, indeed, any IO action performed from main does force evaluation, which should be obvious considering that Haskell programs do, in fact, do things. Anything that needs to go through the sequence defined in main must be in normal form and is therefore subject to strict evaluation.

C. A. McCann got it right in the comments, though: the only thing that's special about main is that main is defined as special; pattern matching on the constructor is sufficient to ensure the sequence imposed by the IO monad. In that respect only seq and pattern-matching are fundamental.

Answer 5

I would probably recast this question as, Under what circumstances will Haskell evaluate an expression? (Perhaps tack on a "to weak head normal form.")

To a first approximation, we can specify this as follows:

• Executing IO actions will evaluate any expressions that they “need.” (So you need to know if the IO action is executed, e.g. it's name is main, or it is called from main AND you need to know what the action needs.)
• An expression that is being evaluated (hey, that's a recursive definition!) will evaluate any expressions it needs.

From your intuitive list, main and IO actions fall into the first category, and seq and pattern matching fall into the second category. But I think that the first category is more in line with your idea of "strictness point", because that is in fact how we cause evaluation in Haskell to become observable effects for users.

Giving all of the details specifically is a large task, since Haskell is a large language. It's also quite subtle, because Concurrent Haskell may evaluate things speculatively, even though we end up not using the result in the end: this is a third breed of things that cause evaluation. The second category is quite well studied: you want to look at the strictness of the functions involved. The first category too can be thought to be a sort of "strictness", though this is a little dodgy because evaluate x and seq x $ return () are actually different things! You can treat it properly if you give some sort of semantics to the IO monad (explicitly passing a RealWorld# token works for simple cases), but I don't know if there's a name for this sort of stratified strictness analysis in general.

Answer 6

A good place to start is by understanding this paper: A Natural Semantics for Lazy Evalution (Launchbury). That will tell you when expressions are evaluated for a small language similar to GHC's Core. Then the remaining question is how to map full Haskell to Core, and most of that translation is given by the Haskell report itself. In GHC we call this process "desugaring", because it removes syntactic sugar.

Well, that's not the whole story, because GHC includes a whole raft of optimisations between desugaring and code generation, and many of these transformations will rearrange the Core so that things get evaluated at different times (strictness analysis in particular will cause things to be evaluated earlier). So to really understand how your program will be evaluated, you need to look at the Core produced by GHC.

Perhaps this answer seems a bit abstract to you (I didn't specifically mention bang patterns or seq), but you asked for something precise, and this is about the best we can do.