## The Etymology of Lifting

This article was originally published in my blog
(affectionately referred to as *blargh*)
on .
The original blog no longer exists as I've migrated everything
to this wiki.

The original URL of this post was at https://tmont.com/blargh/2011/11/the-eymology-of-lifting. Hopefully that link redirects back to this page.

"Lifting" is an esoteric term that is used by a few languages (C♯, most notably) to describe a feature usually associated with nullable types. It's used colloquially to signify that a member (or operator) is "lifted" from another object automatically by the compiler.

What this actually means is best illustrated with an example. C♯'s reference types are nullable by default.
This means you can do things like `object foo = null;`

and not
get a compiler error. It also means you can do things like `foo.ToString()`

and get an ever familiar `NullReferenceException`

.

This is because reference types in C♯ are nullable by default. But not all types are this way. Value types
like `int`

are never null, and have a default value that is retrievable
with the `default` operator: `var zero = default(int);`

The expected behavior becomes less clear when you use nullable value types like
`int?`

. By using nullable types, you can assign a null value to
a value type without a compiler error: `int? nullableInt = null;`

For example, what happens in the following code? Is it a compiler error, a runtime error, or is it valid code that
will run without exception?

```
int? nullableInt = null;
int nonNullableInt = 2;
int result = nullableInt + nonNullableInt; // what happens?
int? result = nullableInt + nonNullableInt; // what about here?
```

The answer I'll leave as an exercise to the reader, as it's mostly irrelevant to what I want to talk about.
That is, the subject of this post is not the peculiarities of the C♯ language specification and the
validity (or lack thereof) of nullable reference types. Both subjects have been hotly debated by people far more
qualified than I. Rather, the subject of this post is what "lifting" means in the context of a programming
language. I gave the example above to illustrate potential ambiguities when performing operations on
nullable types. The actual value of `result`

is insignificant, but the ways in which the compiler
determines how to perform addition between a potentially null value and a non-null value are a good lead-in
to lifting.

## Let's get mathematical

Lifting comes from the prestigious and panty-dropping field of
**topology**, which has to do
with "structured space", which is basically a catch-all term in mathematics. You can hunt down the exact
definition later.

In algebraic topology, there's a thing called a
**homotopy**. Two functions in two
different topological spaces are homotopic if one can be transformed into the other. What that actually means
is not really relevant, but homotopic functions (and in particular homotopies in more than two dimensions)
played a role in proving the Poincaré
Conjecture. So they've served a purpose at some point.

So where does lifting come in? Well, if you have a homotopy on a space **X** to another space
**B** and a mapping
function **δ** from another topological space **E** to **B**,
then **δ** has the *homotopic lifting property* on **X** if it
satisifies some other conditions.

Obviously, that makes no sense, and nor should it, unless you happen to be a grad student studying
algebraic topology. The exact meaning isn't important, but a general inkling of what it represents
will aid in understanding why programming languages borrowed the term *lifting*.

So. The **δ** function above is a map bridging two different spaces. Instead of
spaces, we'll call them sets (since that's actually what
they are). Say **E**
is the set of integers `{ 1, 2, 3, 4 }`

, and **B** is the set of integers
`{ 2, 4, 6, 8 }`

. In this trivial case, **δ** could be $f(x) = 2x$.
It should be pretty obvious to see that **δ** will map each element in **E**
to an element in **B**.

Now, to say that **δ** has the *lifting property* is where it starts to get
interesting. Well, more interesting. Whatever.

Anyway, to say a function has the lifting property requires a bit of verification. Specifically, it requires
several conditions to be true, all of which I'm not going to discuss. You can read about them on wikipedia
if you want. The important thing is that if **δ** has the *lifting property* on a
space **X** (which, remember, contains a homotopy from **X** to **B**), then
there exists another function **g** that maps **X** to **E**.

So, continuing with our trivial example, say **X** is `{ 5, 6, 7, 8 }`

. Then
we could have $g(x) = x - 4$.
This would map all of the elements in **X** to an element in **E**
(which was `{ 1, 2, 3, 4 }`

, as was defined above).

Now, bear in mind that this is all completely contrived and dumbed down for the sake of illustrating
the concept of lifting in programming languages. This is **not** a valid mathematical
notion. This ain't Wolfram|Alpha. For example, **X** must be defined on the continuous
closed interval $[0, 1]$, and there must also exist a map from
$X \times {0}$ to **E**.
But we won't get into that.

So what is happening is that the homotopy from **X** to **B** is "lifted" to
**E** by **g**. What this (approximately) means in layman's terms is that
the homotopy from **X** to **B** can also take place from **X**
to **E**. And what THAT means is a function that can transformed from **X**
to **B** (the definition of homotopy) can also be transfromed from **X**
to **E**.

## Lifting in C♯

So how does all of this relate to C♯ and nullable types? Well, to be able to conveniently perform
addition, C♯ "lifts" the `+`

operator on `int`

to `int?`

. If you think of the set of operators on integers
a topological space, and the set of operators on nullable integers another topological space, the magic
that happens that allows addition between nullable integers would be the **δ** mapping
function that lifts addition to the nullable integers.

I think that might be the worst of analogy of all time. I don't think taking something simple and rewording it be more complex is an actual literary technique. It's possible that I just invented it. But I digress.

Using the term "lifting" is a bit of a misnomer, as it has concrete meaning in other fields (namely algebraic topology). "Lifting" in C♯ is a bastardized form of "lifting" that ignores many of the essential properties of homotopy theory (like the fact that it only applies to continuous functions). But honestly, it doesn't really matter. The visualization of literally (well, figuratively) "lifting" an operator from one object and applying it to another object is pretty apt. But sometimes it's good to understand WHY these things are done this way.