Finish up the draft of the boolean values post.

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Danila Fedorin 2020-08-21 17:37:22 -07:00
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@ -6,7 +6,7 @@ draft: ["Java", "Haskell"]
A friend of mine recently had an interview for a software
engineering position. They later recounted to me the content
of the techical questions that they had been asked. Some had
of the technical questions that they had been asked. Some had
been pretty standard:
* __"What's the difference between concurrency
@ -20,9 +20,9 @@ But then, they recounted a rather interesting question:
> How many values does a bool have?
Innocous at first, isn't it? Probably a bit simpler, in fact,
Innocuous at first, isn't it? Probably a bit simpler, in fact,
than the questions about methods and functions, concurrency
and parallelism. It's plausible that a programmer
and parallelism. It's plausible that a candidate
has not done much concurrent or parallel programming in their
life, or that they came from a language in which functions
were rare and methods were ubiquitous. It's not plausible,
@ -44,13 +44,13 @@ is a property of _something_ that defines what the _something_
means and what you can do with it. That _something_ can be
several things; for our purposes, it can either be an
_expression_ in a programming language (in the form of `fact(n)`)
or a value in that same programming langauge (like `5`).
or a value in that same programming language (like `5`).
Dealing with values is rather simple. Most languages have finite numbers,
usually with \\(2^{32}\\) values, which have type `int`,
`i32`, or something in a similar vein. Most languages also have
strings, of which there are as many as you have memory to contain,
and which have the type `string`, `String`, or occasianlly
and which have the type `string`, `String`, or occasionally
the more confusing `char*`. Most languages also have booleans,
as we discussed above.
@ -124,7 +124,7 @@ But why does _this_ matter? Well, it matters because we don't know
whether or not this function will diverge, and thus, we can't
'throw it out of the window' like we wanted to with `meaningOfLife`!
In general, it's _impossible to tell_ whether or not a program will
terminate; that is the [halting prorblem](
terminate; that is the [halting problem](
So, what do we do?
It turns out to be convenient -- formally -- to treat the result of a diverging computation
@ -187,7 +187,7 @@ expression. What you're doing is a kind of academic autofellatio.
Alright, I can accept this criticism. Perhaps just calling a nonterminating
function a value _is_ far-fetched (even though denotational semantics
_do_ extend types with \\(\\bot\\)). But denotational semantics is not
the only place where types are implcitily extend with an extra value;
the only place where types are implicitly extend with an extra value;
let's look at Java.
In Java, we have `null`. At the
@ -218,3 +218,90 @@ Boolean myTrue = true;
Boolean myFalse = false;
Boolean myBool = null;
Beautiful, isn't it? And, unlike Haskell, where you can't _really_
check if your `Bool` is `undefined` (because you can't tell whether
a non-terminating computation is as such), you can very easily
check if your `Boolean` is `true`, `false`, or `null`:
assert myTrue != myFalse;
assert myFalse != myBool;
assert myTrue != myBool;
We're okay to use `!=` here, instead of `equals`, because it so happens
each boxed instance of a `boolean` value
[refers to the same `Boolean` object](
In fact, this means that a `Boolean` variable can have __exactly__ 3 values!
### C and Integers
Oh the luxury of having a type representing booleans in your language!
It's almost overly indulgent compared to the spartan minimalism of C.
In C, boolean conditions are represented as numbers. You can perhaps get
away with throwing around `char` or `short int`, but even then,
these types allow far more values than two!
unsigned char test = 255;
while(test) test -= 1;
This loop will run 255 times, thereby demonstrating
that C has at least 255 values that can be used
to represent the boolean `true`. There are other languages
with the notion of 'truthy' and 'falsey' values. However,
some of them differ from C in that they also extend this notion
to apply to equality. In JavaScript:
console.assert(true == 1)
console.assert(false == 0)
Then, there are still exactly two distinct values
modulo `==`. No such luck in C, though! We have 256 values that fit in `unsigned char`,
all of which are also distinct modulo `==`. Our boolean
variable can contain all of these values. And there is no
respite to be found with `enum`s, either. We could try define:
enum bool { TRUE, FALSE };
But unfortunately, all this does is define `bool` to be a numeric
type that can hold at least 2 distinct values, and define
numeric constants `TRUE` and `FALSE`. So in fact, you can
_still_ write the following code:
enum bool b1 = TRUE;
enum bool b2 = FALSE;
enum bool b3 = 15;
And so, no matter how hard you try, your 'boolean'
variable can have many, many values!
### Conclusion
I think that 'how many values does a boolean have' is a strange
question. Its purpose can be one of two things:
* The interviewer expected a long-form response such as this one.
This is a weird expectation for a software engineering candidate -
how does knowing about \\(\\bot\\), `undefined`, or `null` help in
creating software, especially if this information is irrelevant
to the company's language of choice?
* The interviewer expected the simple answer. In that case,
my previous observation applies: what software engineering
candidate has _not_ seen a boolean in their time programming?
Surely candidates are better screened before they are offered
an interview?
Despite the question's weirdness, I think that the resulting
investigation of the matter -- outside of the interview setting --
is useful, and perhaps, in a way, enlightening. It may help
one understand the design choices made in _their_ language of choice,
and how those choices shape the code that they write.
That's all I have! I hope that you found it interesting.