# What is unit?

Recall the notion of data structures discussed at the end of this week's further notes on lists. Our (first model of) "cmyg-colors" included four variants, three of which had no parameters and the last of which had two number parameters (one for brightness and another for glossiness). (Subsequently, we altered this to replace one of those parameters with another cmyg-color; but for present purposes let's stick with the first model.)

In the other notes, we discussed how to encode such data structures in the Lambda Calculus. Now let's consider another notation that's sometimes used to talk about them. We might describe such data structures like this:

CMYG_color = Cyan () | Magenta () | Yellow () | Gray (Number, Number)


The component Cyan () says here is one variant, that we will associate with the tag Cyan, and this variant has no parameters. Here we write that as (), for better consistency with the final case where the variant has multiple parameters, but different notational conventions, such as omitting the (), would also be possible. The final component Gray (Number, Number) says the last variant is associated with the tag Gray, and also specifies that this variant has two parameters, and that we expect each of them to be a number. The vertical bar | is just a syntactic separator, like a comma.

These "tags" are also called constructors, or more specifically "data constructors". (Later we will encounter other kinds of constructors. But "constructor" without any modifiers or special context can be assumed to mean these things.) The reason they are called that is that the way to specify some instance of this data structure is to write:

Cyan ()
Gray (5, 0)


These may look like function applications, but they're not. They're canonical constructions of those instances of the data structure. Notice that in the Gray case, we construct an instance using specific numbers; whereas in the description of the data structure itself, we used not specific numbers but instead the general type Number.

(Here too other conventions would be possible, such as, again, omitting the () after the Cyan. Here we also follow the general convention of Haskell and OCaml in capitalizing the alphabetic names of constructors; whereas, on the other hand, variables bound to functions begin with lowercase letters. In fact, in both languages there are also some constructors written outside this convention. Our friends that we express in Kapulet as [] and & can be thought of as constructors, too. Haskell expresses those as [] and : and OCaml as [] and ::. Also, as we said in the other notes, we might think of the boolean truth-values as variants in a data structure. Following the conventions here, the most consistent nomenclature would then be to designate them True () and False (). Haskell sort-of does that, but omits the (). OCaml also omits the () and in this unique case expresses the variants all lower-case rather than capitalized.)

We will be working with all these ideas more in later weeks.

We said that it is possible to think of triples as a data structure, too, that has only one variant with three parameters. We could describe that in Kapulet like this:

... = Triple (Number, Number, Boolean)


Here I just chose specific types for the three parameters. In later weeks we'll see ways to describe triples more generally, without pre-committing to such specific types. We could also write the following in Kapulet:

... = Trio (Number, Number, Boolean)


and if we had both of those, they'd be two distinct data structures, even though they are exactly isomorphic. I mention this only so that you have some idea of why we're choosing the names we are for the variants. The answer is: No reason, it's arbitrary. Different names for the variants would give us distinct but completely isomorphic structures.

Now what about a data structure that has only one variant, that takes no parameters. We could describe that like this:

... = Unit ()


"Unit" is a standard name for this data structure, for the reason that there can be only one instance of the whole structure. In the case of the Boolean structure, there are two possibilities, and in the case of Triples, there are as many possibilities as there are choices of the three parameters. But here there is only one possibility. It may be hard to conceive in advance how or why such a data structure could be useful. But we will see that it is.

In various programming contexts, the word "void" is sometimes used, with varying meanings. Some of those meanings overlap with the notion we're here calling unit. (Though the correspondence isn't perfect or complete.)

So, there are two notions in Kapulet: triples, instances of which we specify as Triple (5, 0, 'false) (or maybe it should be Triple (5, 0, False ())), and unit(s), instance(s) of which can only be specified as Unit ().

As we've observed before, though, Kapulet also has parallel, lighter-weight notion of a tuple or multi-value. In the length-three case, we have the syntax for supplying three arguments at once to an uncurried function, as in f (5, 0, 'false). In the length-zero case we might say instead just g (). (Again, it may be hard to conceive in advance how or why we'd apply a function to no arguments; but read on.) Indeed, it looks like something of the same sort is also going on in our notation Triple (5, 0, 'false) and so on.

It's conceptually helpful to separate some different ideas here, which is why Kapulet has these different notions. One difference is that only the heavier-weight triple is a single value that can be bound to a single variable. So you can do this:

let
x match Triple (5, 0, 'false)
in ...


but you can't do this:

let
x match (5, 0, 'false)
in ...


though you can do this:

let
(x, y, z) match (5, 0, 'false)
in ...


A second, related difference comes up when we consider how these different things embed. If you say:

Pair (Triple (5, 0, 'false), 12)


that Triple is just another value that occupies the first position in the Pair, in the same that the Number 12 occupies the second position. You can also have sequences or sets of such Triples.

On the other hand, you can't say:

Pair ((5, 0, 'false), 12)


Nor could the lightweight tuple be part of a sequence or a set. That's because the lightweight notion is not any single value. It's just syntax for talking about three values simultaneously. You could do this:

Quadruple ((5, 0, 'false), 12)


or this:

let
f match lambda x. (x, 0, 'false)


and those would mean the same as:

Quadruple (5, 0, 'false, 12)


Similarly, Quadruple (5, 0, 'false, (), 12) would also mean the same; though again you may not be able to imagine cases where it's useful to write it in that longer form. On the other hand, you could not say Quadruple (5, 0, 'false, Unit (), 12), because that would be trying to construct a quadruple out of five values.

As I said, there is some conceptual benefit to having both the heavyweight and the lightweight notion of a triple, and the notions of Unit () versus () are the corresponding, limiting cases of these. Our discussion below of the ways in which unit could be useful really applies to both of Kapulet's Unit () and Kapulet's (). It's just that which of these notions you focus on will affect how you conceptualize what's going on. If I say:

g ()


in Kapulet, we should think of that as applying the function g somehow, but to no arguments. That's an odd idea; how is it different from just the function g unapplied? We'll discuss this below. On the other hand, if I say:

h (Unit ())


we should think of that as applying the function h to a single argument, only here it is an argument that is the only possible instance of its data structure. This might seem more comprehensible than g (), but if we can assume that h only accepts arguments from the data structure to which Unit () belongs, it really ought to be just as puzzling. If there's only possible argument that h accepts, then again, how or why should applying h to that argument be different from just the function h unapplied. Indeed, in logic settings this is sometimes how we model constants: as functors from singleton domains.

So we have yet to get a good sense of the usefulness of unit. But in Kapulet at least we're getting some handle on how to manipulate and talk about it/them.

# Other programming languages make the picture messier

Haskell isn't so bad. In Haskell the story is simple: Kapulet's () and (5, 0, 'false) are not part of the language; only Kapulet's Unit () and Triple (5, 0, 'false) are. Awkwardly, though, the way Haskell expresses the latter two notions is like this:

-- Haskell
()
(5, 0, False)


Still, it is just using the same notions Kapulet expresses more verbosely.

In OCaml, things are somewhat as in Haskell. The primary notions are Kapulet's heavier-weight tuples, and they are expressed using sparer syntax:

(* OCaml *)
()
(5, 0, false)


But there are a few odd quirks in OCaml that only make sense if you posit a tacit distinction between these tuples and a notion like Kapulet's lighter-weight tuples (for which OCaml has no explicit syntax). I don't want to detail those quirks now; I'm just saying they are there.

Scheme is much more complicated.

Firstly, Scheme does have notions that systematically parallel Kapulet's lighter-weight tuples. What Kapulet writes as:

f (5, 0, 'false)
g ()


Are in Scheme expressed by:

(f 5 0 #f)
(g)


and Kapulet's:

lambda x. (x, 0, 'false)


most closely corresponds in Scheme to:

(lambda (x) (values x 0 #f))


Following this pattern, Kapulet's () most closely corresponds to Scheme's (values). Scheme's (values 5) evaluates the same as just bare 5 (as does Kapulet's (5)).

Scheme's handling of these multi-value returns is not completely and smoothly integrated into the language though. In later versions of the Scheme standard, they are better-integrated, and there are common extensions to make things even smoother, but it's still not perfect. You can't write things like this:

(f (values 5 0) #f)


and expect it to work the same as (f 5 0 #f). Chicken will just use the first value, 5, and discard any subsequent values. Racket will instead complain, that you gave it two values in a place where it was expecting only one. So although these lightweight multi-values exist in Scheme, they are only occasionally used.

Instead, Scheme generally works with heavier-weight collections. But which ones? Let's focus on triples first (or really any n-tuple, for n at least two), where the answer is already complex. It will become even more so in the case of unit(s).

Scheme has two parallel notions for expressing longer n-tuples. The more straightforward one is called a vector. You can build a vector in Scheme like this:

(vector 5 0 #f)


and Scheme will display the result like this:

#(5 0 #f)


--- perhaps with a further single-quote prefix, depending on your configuration. It's also possible to specify a vector using that latter syntax. Vectors are like tuples in Kapulet, Haskell, and OCaml, in that their different elements can be of heterogenous types. They are like Kapulet's heavyweight tuples in that they can be assigned to single variables, can be discrete elements in other structures, including other vectors, and so on. Scheme vectors are unlike the tuple structures in the other languages in that they are usually mutable: one and numerically the same vector container can contain different elements at different stages in the program's evaluation. But some Scheme implementations also have immutable vectors.

The other notion in Scheme for expressing longer n-tuples is what I'll call the possibly-improper list, or imp. (This isn't standard Scheme terminology. I think it's conceptually cleaner to start here and work your way toward the standard Scheme ways of talking.) I won't say yet how you tell Scheme to construct an imp, but they are displayed like this:

(5 0 . #f)


--- or perhaps with a further single-quote prefix, depending on your configuration. In the special case where the imp is of length two, these are called dotted pairs:

(0 . #f)


If you want to construct an imp without using any variables, then you can do it using the single-quoted version of this:

'(5 0 . #f)


specifies the same length-three imp displayed above. However, if you try this:

(let* ((x 5))
'(x 0 . #f))


you won't get the same thing. Instead, you'll get a length-three imp whose first element is the symbol 'x. Similarly, if you try this:

'(5 (- 3 y) . #f)


you won't get an imp whose second element is 0, even when the variable y is bound to the value 3. Instead, you'll get an imp whose second element is another imp, that begins with the symbol '- and continues with the number 3.

(By the way, in all of these languages the initial position in a sequence is called position 0, the next position 1, and so on. Some languages start counting from 0, others start counting from 1. Nowadays, most do the former. When we use English ordinals in these web pages, though, we will always use "first" to mean the initial position.)

If you try to write simply:

(5 0 . #f)


without a single-quote prefix, that won't work. Scheme will then think you are trying to apply a function (what Scheme calls a "procedure") to several arguments, only when it looks in the head position, it sees the number 5, which is not a function. So it will fail and complain. (You will also be confusing it here with what comes after the ., but I won't explain that.)

There are important semantic generalities about how Scheme works that underwrite this. Consider that the imp you specify by writing:

'(- 3 y)


might, depending on your configuration, be displayed as:

(- 3 y)


But if you submit that, unquoted expression to Scheme, it gets evaluated to the result of applying the subtraction function (or whatever function - is bound to, you can rebind it) to the arguments 3 and whatever value the variable y is then bound to. What is going on here is that the Scheme expression being evaluated is itself an imp. Scheme doesn't think of its programs as flat strings of characters. It thinks of them as already parsed into imps that contain numbers, symbols, and other imps. (You can think of these possibly deeply-embedded imps as constituting syntax trees.) Asking Scheme to evaluate such an imp means for it to apply the function value it gets by evaluating the imp's head, if there be such, to the values it gets by evaluating the other elements in the imp. When you instead specify to Scheme the quoted form:

'(- 3 y)


that refers to the imp itself, rather than to the result of evaluating it in the way just described. Now you see why we call this "quotation", and use the symbol we do for it. You might also see the relation between the symbol 'y (or just y when it occurs embedded in the quoted imp above, or even more deeply embedded, as in '(5 (- 3 y) . #f)) and the variable y. For Scheme, symbols just are variables, only not yet evaluated. That's why we write the symbol with an initial single-quote.

The major difference in Scheme between vectors and imps is that imps have this special relation to Scheme's own syntax. Scheme treats its code as itself being complex imps, not as being complex vectors. Another difference is that under the hood, the computer implements vectors and imps differently. Vectors store all of their elements in a contiguous block of memory (an "array"), whereas imps may store them scattered all over the place (as a "linked list"). On early computing hardware, the latter was often-times more useful; on contemporary hardware, it's much less so. But neither vectors nor imps are closer models of (heavyweight) tuples in Kapulet, OCaml, and Haskell than the other. They can both contain type-heterogenous elements, including other vectors and/or imps. Like vectors, but unlike the structures in other languages, imps in Scheme are by default mutable. But many implementations also offer immutable imps.

Okay, that's the story you should first get your head around about longer Scheme containers. We'll talk about shorter containers, including unit(s), in a moment. But I've used idiosyncratic language in talking about these "imp"s, and I've suppressed some complications, which we should now consider.

One complication is that in the special case where the final element in an imp is Scheme's empty list --- that we can specify as (list) or '() or in some implementations as null (with no surrounding parentheses) --- in that case Scheme calls the imp a (proper) list. (Hence my term possibly-improper list for the general notion.) And in that case the list can be written, not only as:

'(- 3 y . ())


but also as:

'(- 3 y)


with no . and final element. The lack of a . and final element means that the imp's final element is the empty list, and that this possibly improper list is in fact proper. So what Scheme calls its "lists" are in fact a special case of one of its equally-good candidates (vectors and imps) to be identified with Kapulet's, OCaml's, and Haskell's tuples. Scheme doesn't make the sharp distinction between tuples and lists that the other languages do.

We asked before how you specify an imp. We've seen one notation, using an initial single-quote. That works if you're ready to construct the imp without the help of any variables. But if you want to use variables, what should you do? (There is in fact a variation on quotation, called quasi-quotation and using the initial prefix  rather than ' that you could use; but I'm not going to explain that.) If you want to build a proper imp, that is, a Scheme list, you can instead use the function/constructor list:

(list '- 3 y)


returns the imp whose first member is the symbol '- (if you left the quote off, you would instead get the function that this symbol is bound to), whose second member is the number 3, whose third value is whatever value the symbol/variable y is bound to, and whose final value is the empty list.

What if you want to instead build an improper imp, such as (5 0 . y), again using the value y is bound to rather than the quoted symbol? There is no function in standard Scheme to do this directly. But many Scheme implementations do have a special function for it. Racket calls it list*, so if you write in Racket:

(let* ((y #f))
(list* 5 0 y))


You will get the imp whose first member is 5, whose second member is 0, and whose final member is #f (not the symbol 'y).

Other Scheme implementations that provide this function call it cons*, for reasons that will become clearer in a moment.

I said there was no form in standard Scheme to do this directly. You can however do it indirectly, and the reason why brings us to our last complication about Scheme imps. This is that in fact, Scheme imps of length greater than two are really all built up out of embedded "dotted pairs" (that is, imps of length two). The length-three imp '(5 0 . #f) is really implemented in Scheme as a length-two imp, whose first member is 5, and whose second member is another length-two imp, whose first member is 0 and whose second member is #f. It could also be written as '(5 . (0 . #f)). (This is not the same as the imp '((5 . 0) . #f).) Moreover, standard Scheme does have a function/constructor to build the simplest, "dotted pair" case of imps. And what it calls this operation is cons. Yes, the same name we were using to pronounce our list constructor in the other languages. cons does play the list constructor role in Scheme, too, given the way that Scheme in fact implements (what it calls) lists. If ys is bound to one proper list, of any finite length, and y is bound to any value, then (cons y ys) will build a new dotted pair that has those values as elements, and given how Scheme implements proper lists, that just is the longer list whose head is y's value and whose tail is ys's value.

The function that Scheme uses to extract the first element of a dotted pair (and so also to extract the head of a proper or improper list) is car; the function it uses to extract the second element of a dotted pair (and so also the tail of a proper list) is cdr. The reasons for these funny names are historical.

If you grew up learning functional programming and list manipulation from Scheme, all these complexities might seem natural to you, and might shape the way you think about notions like list and ordered pair. Scheme is a great language, but that would be unfortunate. Conceptually, these quirks about Scheme are something of a hack. I think you get a better understanding of the conceptual terrain from the other functional programming languages, and by beginning to think about Scheme's lists and dotted pairs by starting with the notion of a possibly-improper list, rather than the other way around. Most texts teaching Scheme will go in the other direction, though.

Okay, all of that was just us getting clear about Scheme's longer containers, of length at least two. What about the shorter containers?

Scheme does have vectors of length one: you can write (vector 5) or #(5). And that will be distinct from the number 5. Kapulet is similar: we could have a heavyweight one-tuple, perhaps Single 5 (or Single (5), the parentheses make no difference when they contain exactly one syntactic atom), that would be distinct from the number 5. But there is no difference among lightweight tuples. In Kapulet, (5) and 5 would just be notational variants for the number. Although OCaml and Haskell for the most part have tuples corresponding to Kapulet's heavyweight tuples, in this case they do not. Some subtleties about their type systems aside, they don't have any native one-tuple (5) that's distinct from mere 5.

Scheme doesn't have any imps of length one, because it builds its imps out of dotted pairs, so they can't get any smaller than length two. (Some Schemes do have a separate notion of a box, which is isomorphic to vectors of length one.)

Now how about unit(s)? Here again, Scheme has vectors of that length: you can write (vector) or #(). And that will in many ways correspond to the heavyweight Kapulet length-zero tuple Unit (). Here too, there can be no imps that are this short.

But at this point Scheme has yet another special value, that in Scheme documentation is usually called void. And in many Scheme implementations, there is a special function that generates this value, expressed as void. You can apply this function to zero or more arguments; they will all be ignored and you will get the special void value as a result. Generally Scheme doesn't display this value. If you say:

(void)


or:

(let* ((x (void)))
x)


your Scheme interpreter will probably just not show any result, not even a blank line. If you wrote this, however:

(display x)


it might have shown #<void> or #<unspecified>.

Even if your Scheme implementation lacks the void function, though --- as the official standard permits it to do --- you can still generate the special void value in other ways. One example is if a cond expression has no else clause, but none of the clauses that are supplied succeed. So this will also generate the special void value:

(cond
(#f 'impossible))


Ok. Let's pull this all together. Scheme has two heavyweight values corresponding to Kapulet's Unit (), namely its length-zero vector (expressible as (vector) or #()) and its special void value (expressible in various ways). It has two heavyweight values corresponding to Kapulet's Triple (0, 5, #f), namely a length-three vector and a length-three imp. Corresponding to the lightweight Kapulet tuples or multi-values are the Scheme idioms:

; when calling Scheme functions
(f 0 5 #f)
(g)


and:

; when returning from Scheme functions
... (values 0 5 #f)
... (values)


I told you the correspondences between all these languages was complex. But now, finally, you should have some handle on how to manipulate and talk about unit(s) in languages besides Kapulet. We still have to figure out what these are good for.

# Using unit as a parameter in a data structure

As I mentioned in class, you might sometimes want to use unit as a parameter to some existing data structure. For instance, sequences or lists are data structures that have one variant [] with no parameters, and a second variant with both a head parameter and a tail parameter that has to be another list. What is the head parameter? Well, in some cases it might be a Number, or a Boolean, or another atomic symbol. Or even a function. But in some cases we might not care about the specific identity of the head. We might only care about the list structure. (For lists, all there is to their structure is their length. For other structures, their structure might be more complex and encode more information.) We could just make a list of Numbers, and ignore the identity of the particular Numbers used. But it's ugly to make arbitrary choices about which Numbers to use, when we really don't care. It can also mislead readers of our code about how the code works. If we want to specify clearly that we don't care about the identity of the head element, we could instead make it a list of Units, where there is only ever one choice for which instance of that type to use.

If the notion so expressed is important enough, we might give it its own, dedicated data structure, that just left out the head parameter. Informationally, it's all the same whether you omit some parameter or include it but offer only one choice for what it can be. Indeed I suggested that this is a helpful way to think of Numbers as compared to Lists. (In Chapter 6 of The Little Schemer, they also briefly explore representing the number 4 as the list '(() () () ()).)

But sometimes the more general data structure you're working with will be well-developed, and have lots of code already built up around it, prepared to handle parameters of many types. And the special case where you don't care about the identity of the parameter might be more limited purpose, that's easier to just piggy-back on the more general notion than to write separate code for. In these cases Units can be a useful choice for the type of some parameter, precisely because they are uninformative.

(For these purposes you'd want to use heavyweight units, like Kapulet's Unit () or Haskell and OCaml's (). For the remaining jobs to be discussed below, however, arguably either of Kapulet's heavyweight or lightweight units could be deployed.)

# Returning unit as a result

Some functions return Units as their results. How could that ever be useful? Since there's only ever one result, it seems like the function would have to be a constant function, from whatever arguments it was willing to accept, to that result. Now sometimes constant functions might be useful; for example, you might have an operation that expected some predicate to apply to numbers, and in a particular case you might want every number to count as passing, so you could supply the constant predicate from numbers to truth. But here the constant function's usefulness depends on the possibility of your choosing it rather than some other function with the same result type. Even if you limited yourself only to constant functions to Boolean results, you still have two to choose from, and that choice can encode some information. When we turn to constant functions from some arguments into the Unit type, on the other hand, there is only one result that any such function could constantly return. What would be the point?

The point emerges when we have functions that do more than merely evaluate other functions and relations on their arguments. Some functions additionally perform side-effects, like vocalizing (or printing) their arguments, or changing what the first element in some mutable list value is, or so on. A function can perform side-effects and also return a useful value. For example, we might have a "noisy-successor" function that vocalizes the value of its number argument, and evaluates to that argument's successor. So if I ask the computer to evaluate:

[noisy_succ 3, 5]


or:

[noisy_succ (1 + 2), 5]


The computer will vocalize the word "three", but return the value 4, to be used in building up a potentially more complex value --- in these cases the length-two sequence [4, 5].

For some functions, on the other hand, all we care about is the side-effect. There may be no informative result to return. Then we're in a situation like we were in the previous section. We could return, say, a Number result, chosen arbitrarily. Or a random Boolean. But this is ugly and can mislead readers about what's going on. The cleanest thing to do is to signal explicitly that the result value is not informative. For this Units are perfect.

We haven't yet started to work with any functions with side-effects, but we will later. You may also be familiar with some of these ideas from other contexts, or other programming languages.

# Supplying unit as an argument

Finally, in some cases we have functions that are applied to Units as arguments. How can that be useful? What's the difference between applying a function to an uninformative argument and just the original, unapplied function?

As Chris observed in seminar, we do make such distinctions in natural language. We distinguish between the English predicate "is raining" and the sentence "It is raining." Only the latter can have a truth-value (in a particular context of utterance, that supplies a time and a place). Only the former still has syntactic positions left unsaturated, and can be coordinated with other, similar predicates ("is raining and won't get warmer"). Yet that initial "It" is semantically uninformative. It doesn't refer to anything. All it does is provide empty filler. It seems like its sole function is to mark the difference between the predicate and the sentence.

This is indeed the role unit has when it's the argument a function expects or receives. Now, you ask: why would it be useful to distinguish between the unapplied and the applied function. If one Kapulet programmer writes:

let
g match lambda (). 1 + 3;
f match lambda (thunk, y). thunk () * y
in f (g, 5)


and another writes merely:

let
g' match 1 + 3;
f' match lambda (x, y). x * y
in f' (g', 5)


what advantage can the first have possibly achieved? Why bind g to such a function --- these functions that take () arguments are known as thunks --- which later gets applied, but to an uninformative argument, rather than just calculating the function's result in the first place, and binding a variable that that?

The main usefulness of this is again when we're dealing with functions that have side-effects. If g were bound instead to lambda (). noisy_succ 3, we might want to control when the body of that function gets computed. We might be in a position to build the function at one point in the program, but only want it to be computed at some distant point, related to where we are now by a complex path. Also, we might want to control how often the body gets computed. If for example, we say:

let
g match lambda (). noisy_succ 3;
f match lambda (thunk, y). thunk () * thunk () * thunk () * y
in f (g, 5)


that will result in the computer vocalizing "three" three times, and returning 320. Whereas if we say:

let
g' match noisy_succ 3;
f' match lambda (x, y). x * x * x * y
in f' (g', 5)


that will result in the computer vocalizing "three" only once, and returning the same result. (I assume here our language has what we called "strict" or "eager" evaluation order, where a function's arguments are evaluated before being substituted into the body of the function. Most programming languages work like this; but Haskell does not do so by default. With the Lambda Calculus, it may or may not; you have to decide.)

Here again, we haven't started to work with any functions with side-effects, so you have to imagine ahead ways in which this could be useful.

But in this case, we have also encountered this very week another way in which delaying the evaluation of a function in this way could be useful. Some expressions just won't evaluate to any value. In the Lambda Calculus, we had the example of ω ω, that is, (\x. x x) (\x. x x). And also some even scarier terms. We can write these in Kapulet, or we can write other scary terms using letrec:

letrec
blackhole match lambda (). blackhole ();
fst match lambda (x, y). x
in fst (5, blackhole)


If blackhole ever gets applied to its () argument, computing the result will require re-applying blackhole to that same argument, which will require ... and the reduction or computation will never terminate. Some of the vocabulary people use for such expressions is that they involve "infinite loops" or "non-termination" or that they "diverge" or that their "meaning is bottom" (written ⊥, as we sometimes use in logic to represent a constant formula that is always false). "Bottom" is a meaning we might assign these terms, but don't say that this is their value. Such terms don't have any value. (Sometimes people talk sloppily, and we might even do it ourselves. But the best way to talk here is to say that expressions like blackhole () don't have any values, or in other words don't evaluate to anything, though our semantics may assign them a meaning or denotation. Interestingly, you can't in general assign all non-terminating expressions the same meaning. See ___ for discussion.)

But in the complex expression above, we never do apply blackhole to an argument, so no harm comes about. It makes no difference that we were evaluating fst (5, blackhole) rather than snd (blackhole, 5) (or even snd (5, blackhole)). In none of the cases do we ever request the result of computing blackhole's body. So everything is ok.

This is like how in the Lambda Calculus it can be alright to say:

K I (ω ω)

--- that is, assuming the lambda term is being evaluated in "normal" or "lazy" order, so that we reduce the leftmost, outermost redex K I (...) before we reduce the argument redex ω ω. Since K I (...) discards its second argument, the non-terminating computation of ω ω` (that is, the result of self-applying self-application) is never demanded.