Strictness and how to tell GHC/GHCi to just store a value in a variable once and for all

Apologies if this is a common question, I couldn't find anything similar, but I may just be too inexperienced to know the proper vocabulary.

Here's an example of the fundamental problem in GHCi:

-- foo is something relatively expensive to compute, but it's lazy so it's instantaneous
*Main> foo = (foldl (++) [] (take 5000 $ repeat [10, 123, 323, 33, 11, 345, 23, 33, 23, 11, 987]))
(0.00 secs, 62,800 bytes)
-- bar is a result that uses foo, but it's also lazy so it's not computed yet
*Main> bar = last foo
(0.00 secs, 62,800 bytes)
-- Now let's use bar
*Main> bar
(1.82 secs, 11,343,660,560 bytes)
-- It took a couple seconds to compute everything, but that's fine. Now let's use it again:
*Main> bar
(1.88 secs, 11,343,660,560 bytes)
-- It took 1.88 seconds to presumably recompute everything whereas it
-- could have been instantaneous if GHCi had remembered the value.

I looked into strictness, but nothing I do seems to help. As I understand it, evaluating an argument strictly should cause all computation to be done immediately, yet neither seq nor $! seem to enable this behavior. For instance:

-- expectation: Perhaps evaluating bar will cause 'last foo' to be run systematically,
-- but foo should be evaluated strictly, which should take most of the computation time 
*Main> bar = last $! foo
(0.00 secs, 62,800 bytes)
-- it's actually instantaneous. Sure enough, bar takes time to compute.
*Main> bar
(1.88 secs, 11,343,660,632 bytes)
-- what if we use seq? As I understand it, it returns its second argument
-- with the constraint that its first argument must be strict. We want
-- 'bar = last foo' with foo being strict, therefore:
*Main> bar = foo `seq` (last foo)
(0.00 secs, 62,800 bytes)
*Main> bar
(1.93 secs, 11,344,585,344 bytes)
-- No dice. What if we evaluate bar strictly and assign that value to a variable 'baz'?
*Main> bar = last foo
(0.00 secs, 62,800 bytes)
*Main> baz = bar `seq` bar
(0.00 secs, 62,800 bytes)
*Main> baz
(3.80 secs, 22,689,057,424 bytes)
-- Now it's computing it twice! Once for the `seq` and then a second time to display the value?
-- What if we use BangPatterns? Let's just put a variable through a dummy
-- function that returns its own input (hypothetically) after evaluation.
*Main> eval !x = x
(0.00 secs, 62,800 bytes)
*Main> baz = eval bar
(0.00 secs, 62,800 bytes)
*Main> baz
(1.81 secs, 11,345,102,464 bytes)
-- still not doing the computation ahead of time.

So It looks like I fundamentally don't understand what 'strictness' is, and I still can't get GHCi to store anything but thunks into variables. Is this just a quirk of GHCi, or does this apply to GHC? How do I get my code to just store a normal-ass value like 987 in a variable?


  • There are a few different things going on here, unfortunately.

    Type-class polymorphism

    Joseph Sible's answer pointed this out, but I think there are a few more details it's worth covering. If you type x = 1 + 1 into GHCi, x will have type Num a => a; it's polymorphic, and the type variable has a constraint on it. At runtime such values are represented as a function, with a parameter that is basically the Num instance for the type you want to use (the runtime instance is called a "dicitonary").

    This is necessary because you could type print (x :: Integer) and GHCi has to figure out what value 1 + 1 is for the Integer type, or print (x :: Complex Double) and the compiler has to figure out what 1 + 1 is for the Complex Double type; both of those types use a different definition of + (and of the fromInteger function that says what integer literals mean). You can even create a wholly new type after defining x, give it a Num instance, and then GHCi still has to be able to print (x :: MyNewNumType). All of that basically means that x :: Num a => a cannot be evaluated once-and-for-all when you define it; instead it is stored as a function, and every time you use it as some particular type it gets evaluated again, so that its definition can use the + and fromInteger functions appropriate to the type at this usage.

    This is typically not a huge problem in "normal" Haskell, written in modules rather than entered into the GHCi prompt. In normal Haskell the compiler will be able to analyse the full module to see where x is used, so it will often be able to infer a particular type for x (i.e. if somewhere else in your module you use x to index into a list with the !! operator, that only accepts Int as the index, so x will be inferred as type Int, not as the more generic Num a => a). And if that fails because none of the usage sites in the module require x to be a specific type, then there is also the monomorphism restriction that will force x to be assigned a concrete type anyway, invoking the defaulting rules, which would pick Integer in this case. So normally a definition will only end up with a type like Num a => a if it specifically has a type signature requesting that type. (The monomorphism restriction applies to bindings of bare variables without any function arguments, so it will apply to x = 1 + 1, and f = \x -> x + 1, but not to f x = x + 1)

    But the monomorphism restriction is disabled in GHCi. It was in fact specifically added to the language to avoid performance problems due to type-class polymorphic values being recomputed on every use instead of cached. But it only produces sane results when applied to entire modules where the compiler can see usage of the variables to assign a correct monomorphic type. In GHCi where you enter things one line at a time, forcing the compiler to pick a monomorphic type for bindings like x after seeing only the definition worked extremely poorly; it would very often assign the wrong type for the usage you intended, then generating spurious type errors when you tried to use it. So eventually the monomorphism restriction was disabled by default in GHCi, making it much more usable but having these performance issues of values being recomputed more than necessary.

    So this issue is mostly a problem for you because you're working in GHCi. In "normal" Haskell it still can happen, but default settings and good practice (like giving type signatures to all your top-level bindings) make it a very minor concern in practice.

    Haskell is timeless

    This is an extremely common mistake when first learning about applying strictness in Haskell, so you're in good company! I'm going to talk about seq here, but the same concepts apply to things like $! and bang patterns; for example f !x = (x, x) is basically just a convenient shorthand for f x = x `seq` (x, x).

    baz = bar `seq` bar isn't doing what you think it's doing. In fact it isn't doing anything at all, it's equivalent to simply baz = bar.

    The reason is that seq doesn't cause evaluation now, and the reason for that is not some quirky technical thing, it's that there is no such thing as "now" in Haskell. This is the fundamental difference between a pure declarative language and an imperative language. Imperative code is a sequence of steps that are executed in order, so the notion of time is inherent to the language; there is a "now" that "moves through" the code. Pure declarative code isn't a sequence of steps, it's set of definitions. There's no "now" because there's no "time"; everything just is. baz = bar `seq` bar is a definition for what baz is, not a step to execute; there's no "before" and "after" to define the "now" when seq could cause bar to be evaluated.

    Moving away from this concept of code being inherently tied to a notion of time is one of the trickiest mental leaps imperative programmers need to make when they come to a pure language like Haskell for the first time (and in a pure but strict language you don't necessarily even need to; it's still reasonably easy to mentally model such code as a sequence of steps, even if I would argue it's still not the best conceptual model). And it's complicated by GHCi because the interpreter obviously does have a notion of time; you enter bindings one at a time, altering the set of definitions that the interpreter is aware of. In normal module Haskell that set of definitions is static but in GHCi it changes over time, inherently defining a "now". But seq is a feature of Haskell, not of GHCi, so it has to make sense without GHCi's notion of time. Even though the set of definitions changes over time in a GHCi session, those definitions themselves still don't have any notion of time; they are written in Haskell, which is timeless.

    So without any notion of time seq can't actually control when something is evaluated. What it does instead is control what gets evaluated. Evaluation in Haskell is driven by need; if x is being evaluated and x is defined by x = a + b, then the evaluation of x imposes a need to evaluate a and b (and +; technically + is what immediately is needed and the definition of + will decide whether a and/or b is needed, but for most types' definitions of + both will end up needed). All seq does is add additional "need". In x = a `seq` b, x is defined to be equal to b (because that's what seq returns), so evaluating x will obviously need to evaluate b; the special thing about seq is that it says that evaluating x will also need to evaluate a (and it can do this without knowing anything about a or b). But if we never needed to evaluate x itself then the fact that seq is used in the definition of x won't do anything.

    In GHCi's notion of time, x = a `seq` b doesn't evaluate a "now" when that binding is entered. It will evaluate a if x is ever used (in a manner that requires it to be evaluated), which will be after some later entry at the command prompt.

    This is how we come to baz = bar `seq` bar just being equivalent to baz = bar. baz = bar `seq` bar means that when baz is evaluated, we will also need to evaluate bar. But baz is equal to bar, so of course we're going to need to evaluate bar by definition!

    Similarly bar = foo `seq` (last foo) doesn't help because evaluating bar required evaluating last foo, which is going to have to evaluate foo anyway.

    So this is also sort-of an issue that is more of a problem in GHCi than in normal Haskell. In GHCi it's very easy to think in terms of the "now" that is you entering things into the command prompt, and thus have a clear idea in your head of what "evaluate this now" would mean and expect that adding strictness would do that. In normal Haskell it's still easy for beginners to make that mistake, but it's less unclear that the notion of "now" doesn't make much sense and thus hopefully easier to develop better expectations on what seq is able to do.

    But we can use time when we have it

    There is a quick hack you can use to exploit GHCi's notion of time, though.

    If I enter b = not True into GHCi (an example avoiding numbers to not trip over the type class polymorphism complication), then b is defined but it's not evaluated yet. Using strictness (via whether via seq, $!, or anything else) inside the definition of b won't help, because it can only make extra things be evaluated when b is evaluated, not change the fundamental reality that merely defining b doesn't evaluate b to trigger any of that extra strictness.

    But after I've defined b I can immediately enter a new command that will cause b to be evaluated. In this case a simple b would work fine, since GHCi printing it will inherently force evaluation. But if I didn't want to actually print b, I could enter this:

    b `seq` ()

    This tells GHCi to print (), but to do so it first has to evaluate b (without doing anything else with it).

    The other thing that can give us a notion of time is IO. IO code is still Haskell code, which means it's technically "timeless". But the interface of IO is designed to model (in timeless Haskell), the things that can be done by interacting with the real world. The real world is definitely not timeless, so that notion of time inherently enters the picture of IO code; effectively the data dependencies of an abstract monad construct a notion of time out of Haskell's timeless semantics, and IO just hooks up execution with the real world so that the constructed time matches up with real world time. This means that a function that means "evaluate this now" does make sense in IO, and indeed there is the function evaluate :: a -> IO a (you have to import it from Control.Exception though). This is using IO's notion of time rather than GHCi's, so this even can be used in module Haskell! But it of course only works in IO code.

    So using evaluate bar as a command in GHCi will also work as a way to evaluate bar "right now". (evaluate returns the value though, so GHCi will print it; if you don't want that then bar `seq` () would still be preferable)

    You can even incorporate evaluate into the definition of a variable in GHCi to make it evaluated as it is defined, but you have to use the monadic binding <- instead of normal definition with = (exploiting the fact that the GHCi prompt is more-or-less "inside IO").

    λ b = not True
    b :: Bool
    λ :sprint b
    b = _
    λ b
    it :: Bool
    λ :sprint b
    b = False
    λ c <- evaluate $ not True
    c :: Bool
    λ :sprint c
    c = False

    (Note here I'm using the :sprint GHCi special command; instead of converting a whole value to a string with show, :print and :sprint print out a string intended to show the structure of the value without forcing any evaluation; :sprint just shows _ where an unevaluated value is found, while :print also includes information about its type. These can be very handy for checking whether other commands have caused evaluation; it's more precise than relying on the time it takes for a command to be processed. You have to use them on a variable though, not an expression)

    Similarly some other monads like State etc can also be viewed as building a notion of time on top of Haskell's timeless semantics, but they don't allow truly arbitrary side effects like IO does, and their constructed timelines don't necessarily match up with the real world's time since they are evaluated whenever timeless lazy Haskell needs (and thus might be run zero or multiple times); they're more of a simulation of time. So an "evaluate this now" function in a "now" derived from those timelines wouldn't be as useful.

    Weak head normal form

    Finally, there is still a problem with trying to strictly evaluate foo. Say you use a type annotation to avoid the polymorphism and use an extra foo `seq` () command to evaluate it, like this:

    λ foo = (foldl (++) [] (take 5000 $ repeat [10, 123, 323, 33, 11, 345, 23, 33, 23, 11, 987 :: Integer]))
    foo :: [Integer]
    λ foo `seq` ()
    it :: ()

    This hasn't actually evaluated "all" of foo:

    λ :sprint foo
    foo = 10 : _

    We've only evaluated up to the first element of the list! That's probably not what you wanted.

    So far in this post we've just been talking about a value being evaluated without any clarification, like that's a single atomic step. That's true for very simple values like Bools, Ints, Chars, etc. But most values (like lists) have more structure than that, potentially containing many sub-values that can be evaluated or not. So when we say that such a value is being evaluated, there are many possibilities for what we could mean.

    In Haskell the "standard" notion of evaluation is always to weak head normal form. That has a fancy technical definition, but basically it means evaluated until we have the outermost data constructor (like the : for lists). This is because most of the "need" in Haskell's need-driven evaluation comes from pattern matching; at minimum the system will need to know what the outermost data constructor is to decide which branch to take in pattern matching (e.g. many list functions will have a case for [] and a case for :). All the values contained in that outermost constructor (e.g. the head and tail of the list) will be left unevaluated if they haven't already been evaluated by something else, until more pattern matching needs to examine those contained values.

    So when we say that foo `seq` () is equivalent to a () but with additional need to evaluate foo, evaluating the () only causes foo to be evaluated as far as the very first list cell (it doesn't even evaluate the value inside that list cell, but because it ultimately came from a literal in your source code it was already evaluated, which is why :sprint showed 10 : _ and not _ : _).


    The module Control.DeepSeq has tools for forcing deep evaluation. For example there is deepseq that can be used in the same way as seq, but it forces the left argument to be completely evaluated. There is also force that can be used in combination with evaluate to force deep evaluation "now" (using IO's notion of "now").

    This is all based on a type class NFData though (the NF in the name is short for "normal form"; normal form is fully evaluated, while weak head normal form is evaluated to the outermost constructor). You can't deepseq a value of a type that doesn't implement the class. Most standard types already have instances, so this works fine for foo :: [Integer]. But if you're using your own types you will need to give them instances of NFData.

    So this will actually define foo and then fully evaluate it (without printing it):

    λ foo = (foldl' (++) [] (take 5000 $ repeat [10, 123, 323, 33, 11, 345, 23, 33, 23, 11, 987 :: Integer]))
    foo :: [Integer]
    λ foo `deepseq` ()
    it :: ()

    But deep evaluation inherently involves a full traversal of the data (you can't evaluate everything without looking at everything). So using deepseq a lot can be very inefficient; if you were going to evaluate it later anyway you're wasting time doing an additional pass over the data (and if you weren't going to use it later then you're forcing the system to spend time running code it could have skipped entirely).

    It's useful when you have a computation depending on large structures that will result in only a small structure (but bigger than a single number, or else seq would have been enough to force it); if it would otherwise be left only partially evaluated for a long time, deepseqing it might avoid having to keep the large structure in memory at all, which can be more efficient (even though you've added an extra small pass over the small result structure).

    It's probably more commonly used though when the computation might throw an exception (or has side effects smuggled in with unsafePerformIO et al); deepseqing it will force those exceptions or side effects to manifest at a known point, instead of whenever later code happens to need a specific part of the result. This is where the evaluate . force combo comes in particularly useful, since you're forcing it in IO where you have a well-defined "now" and also can handle exceptions.

    But generally deepseq is better avoided if you can. It's an important tool to have available for sparing use, not a way of making Haskell behave exactly like strict languages.