# 🗺 A tour of Unison

This document walks through the basics of using the Unison codebase manager and writing Unison code. We will introduce bits and pieces of the core Unison language and its syntax as we go. The Unison language documentation is a more in-depth resource if you have questions or want to learn more.

If you want to follow along with this document (highly recommended), this guide assumes you've already gone through the steps inthe quickstart guideand skimmed through

## 👋 to the Unison codebase manager

The Unison Codebase Manager, or UCM for short, is the command line tool that runs the Unison programming language and allows you to interact with the Unison code you've written and saved. Put differently, the UCM is the interface to your Unison codebase.

💡 Remember: Unison code is not saved as text-based file content. Because of this, we need a tool that lets us change and run Unison programs.

Its many responsibilities include:

• Typechecking and compiling new code
• Organizing, navigating, and finding Unison definitions
• Storing the state of your codebase
• Running Unison programs and Unison binaries
• Publishing and pulling Unison libraries

## 🎉 Running the UCM

By default, runningucmin a directory will interact with any.usuffixed file in the directory where the command was issued while opening the default codebase in your home directory. You'll get a message in the UCM like:

Now starting the Unison Codebase Manager (UCM)...

[…]

🐣 Since this is a fresh codebase, let me download the base library for you.

What's happening here? This is the Unison Codebase Manager starting up and initializing a fresh codebase with the standard library. We're used to thinking about our codebase as a bag of text files that's mutated as we make changes to our code, but in Unison the codebase is represented as a collection of serialized syntax trees, identified by a hash of their content and stored in a collection of files inside of a.unisondirectory in the path you supplied to the ucm.

The Unison codebase format has a few key properties:

• It isappend-only:once a file in the.unisondirectory is created, it is never modified or deleted, and files are always named uniquely and deterministically based on their content.
• As a result, a Unison codebase can be versioned and synchronized with Git or any similar tool and will never generate a conflict in those tools.

Let's explore thebaselibrary that was just downloaded and get used to navigating a Unison codebase.

You can view the terms and types in a namespace with thelsucm command.

.> ls base.data.List

The output should be a numbered list of definitions and their associated signatures.

.> ls base.data.List

1.   ++                    ([a] -> [a] -> [a])
2.   +:                    (a -> [a] -> [a])
3.   :+                    ([a] -> a -> [a])
4.   Nonempty              (type)
[…]
📁
Here the command is performed in the top-level namespace, represented by.It's common for the top-level of your codebase to house multiple Unison namespaces representing the various projects you're working on. The prompt shows us which namespace we are currently in. If we were in a different namespace, we would need to change thelscommand from using therelative pathbaseto theabsolute path.base.

Because of the append-only nature of the codebase format, we can cache all sorts of interesting information about definitions in the codebase andnever have to worry about cache invalidation.For instance, Unison is a statically-typed language and we know the type of all definitions in the codebase—the codebase is always in a well-typed state. So one thing that's useful and easy to maintain is an index that lets us search for definitions in the codebase by their type. Try out the following commands (new syntax is explained below):

.> find : [a] -> [a]

1. base.data.List.distinct : [a] -> [a]
2. base.data.Heap.sort : [a] -> [a]
3. base.data.List.dropLast : [a] -> [a]
4. base.data.List.reverse : [a] -> [a]
5. base.data.Heap.sortDescending : [a] -> [a]

.> view 4

base.data.List.reverse : [a] -> [a]
base.data.List.reverse as = List.foldLeft (acc a -> a +: acc) [] as

Here, we used the did a type-based, withfindfollowed by a colon,:,to search for functions of type[a] -> [a].We got a list of results, and then used theviewcommand to look at the nicely formatted source code of one of these results. Let's introduce some Unison syntax:

• List.reverse : [a] -> [a]is the syntax for giving a type signature to a definition. We pronounce the:symbol as "has type", as in reverse has the type[a] -> [a].
• [Nat]is the syntax for the type consisting of lists of natural numbers (terms like[0, 1, 2]and[3, 4, 5],and[]will have this type), and more generally[Foo]is the type of lists whose elements have some typeFoo.
• Any lowercase variable in a type signature is assumed to beso[a] -> [a]really means and could be writtenforall a . [a] -> [a],which is the type of functions that take a list whose elements are some (but any) type, and return a list of elements of that same type.
• List.reverse astakes one parameter, calledas.The stuff after the=is called thebodyof the function, and here it's awhich is demarcated by whitespace.
• acc a -> ..is the syntax for an anonymous function.
• Function arguments are separated by spaces and function application binds tighter than any operator, sof x y + g p qparses as(f x y) + (g p q).You can always use parentheses to control grouping more explicitly.
Try doingview base.data.List.foldLeftif you're curious to see how it's defined.

### 🖼 Check out the local codebase UI

We've been navigating the codebase via the UCM command line, but there's another option for exploring and viewing a Unison codebase: the Unison codebase UI.

The codebase UI is a graphical interface for exploring your codebase. You can search for terms, click-through to Unison code definitions, and read code documentation.

Typeuiin the UCM to open the local UI and search forText.dropRightWhile.Unison docs are automatically linked to the term and the source code is available for exploration.

### Names are stored separately from definitions so renaming is fast and 100% accurate

The Unison codebase, in its definition forList.reverse,doesn't store names for the definitions it depends on (like theList.foldLeftfunction); it references these definitions via their hash. As a result, changing the name(s) associated with a definition is easy.

Let's try this out.List.reverseis defined usingList.foldLeft.Let's rename that toList.foldlto make it more familiar to Haskell fans. Try out the following command (you can use tab completion here if you like):

.> move.term base.data.List.foldLeft base.data.List.foldl

Done.

.> view base.data.List.reverse

base.data.List.reverse : [a] -> [a]
base.data.List.reverse as =
use base.data.List +:
base.data.List.foldl (acc a -> a +: acc) [] as

Notice thatviewshows thefoldlname now, so the rename has taken effect. Nice!

To make this happen, Unison just changed the name associated with the hash ofList.foldLeftin one place.Theviewcommand looks up the names for the hashes on the fly, right when it's printing out the code.

This is important: Unisonisn'tdoing a bunch of text mutation on your behalf, updating possibly thousands of files, generating a huge textual diff, and also breaking a bunch of downstream library users who are still expecting that definition to be called by the old name. That would be ridiculous, right?

So rename and move things around as much as you want. Don't worry about picking a perfect name the first time. Give the same definition multiple names if you want. It's all good!

The fact that Unison codebases are immutable and append-only means that we can "rewind" our codebase to an earlier point in time. Use thereflogcommand to see a log of the codebase changes. You should see some help text and a numbered list of hashes.

1. #2cbugd57qa : move.term .base.data.List.foldLeft .base.data.List.foldl
2. #na6fel77ai : pull unison.public.base.latest .base
3. #sjg2v58vn2 : (initial reflogged namespace)

Reflog keeps track of the history of the codebase by recording the hash of the rootnamespaceof your codebase. Namespace hashes change along with updates to the term and type definitions that they enclose. When we renamedList.foldLeft,conceptually, the "state" of the codebase changed, but the log-based format of the codebase history means those changes are retrievable.

Let's try to undo the rename action. Use thereset-rootcommand to pick a prior codebase state to return to. We'll give it the hash of the codebase from just before themove.termcommand was issued.

.> reset-root #na6fel77ai

Done.

Great! OK, go drink some water, 🚰 and then let's learn more about Unison namespaces and the expected codebase structure.

## Namespace conventions and searching the UCM

Thus far we've been exploring thebasestandard library that automatically downloads into the root of your codebase, but when you're writing your own Unison code, you won't want to add your functions and terms at the root. Instead, let's introduce the concept of "namespaces" by creating atournamespace and establishing some conventions for organizing it.

Unisonnamespacesare mappings from names to definitions. Names in Unison look like this:math.sqrt,.base.Int,base.Nat,base.Nat.*,++,orfoo.That is: an optional.,followed by one or more segments separated by a.,with the last segment allowed to be an operator name like*or++.

We often think of these names as forming a tree, much like a directory of files, and names are like file paths in this tree.Absolute names(like.base.Int)start with a.and are paths from the root of this tree andrelativenames (likemath.sqrt)are paths starting from the current namespace, which you can set using thenamespace(or equivalentlycommand:

In the codebase manager console, create atournamespace with thecdcommand. This is where you'll adding code for the remainder of this guide.

.> cd tour

Notice the prompt changes to.tour>,indicating your current namespace is now.tour.When editing Unison code, and interacting with the UCM any relative names not locally bound in your file will be resolved by prefixing them with the current namespace of.tour.Your UCM commands and code are "scoped" to this namespace unless otherwise indicated with an absolute path.

Next, create a copy of thebasedirectory byforkingthe top-levelbaselibrary into a namespace calledlib.

.tour> fork .base lib.base
🧠
Unison expects namespaces directly under the root, like ourtournamespace, to have one namespace calledlibcontaining all the dependencies necessary for running the code in the namespace tree.

Earlier, when we used thefindcommand, we were searching from the root of our codebase, but within thetournamespace, which might grow to contain multiple libraries, we'll want a different set of conventions for finding our own code versus searching through our dependencies.

Try using thefindcommand again forList.reverse:

.tour> find List.reverse

I couldn't find matches in this namespace, searching in 'lib'...

1. lib.base.data.List.reverse : [a] -> [a]

When we'reinside the tour namespace,the UCMfindcommand will search for non-dependency Unison code in the current namespace tree before searching through the lib directory.

If you want to perform a search which specifically includes thelibnamespace, you can use thefind.allUCM command.

.tour> find.all List.reverse

1. lib.base.data.List.reverse : [a] -> [a]

Notice how these results don't contain the top levelbaseresult? That's becausefindandfind.allare both scoped to thetournamespace based on where we're searching!

If we want a global search of our codebase, we can use thefind.globalcommand.

Ok, that's enough preamble, let's start writing some Unison code!

### Unison's interactive scratch files

The codebase manager lets you make changes to your codebase and explore the definitions it contains, but it also listens for changes to any file ending in.uin the current directory. When any such file is saved (which we call a "scratch file"), Unison parses and typechecks that file. Let's try this out.

Keep yourucmterminal running and open up a file,scratch.u(orfoo.u,or whatever you like) in your preferred text editor (if you want syntax highlighting for Unison files,follow this linkfor instructions on setting up your editor).

Now put the following in your scratch file:

use base
square : Nat -> Nat
square x =
use Nat *
x * x

This defines a function calledsquare.It takes an argument calledxand it returnsxmultiplied by itself.

The first line,use base,tells Unison that you want to use short names for the base libraries in this file (which allows you to sayNatinstead of having to saybase.Nat).The UCM will prefer thebaseinstance found inlib.

When you save the file, Unison replies:

✅

I found and typechecked these definitions in ~/unisoncode/scratch.u. If you do an
add or update , here's how your codebase would change:

⍟ These new definitions are ok to add:

square : base.Nat -> base.Nat

Now evaluating any watch expressions (lines starting with >)… Ctrl+C cancels.

It typechecked thesquarefunction and inferred that it takes a natural number and returns a natural number, so it has the typeNat -> Nat.It also tells us thatsquareis "ok to add." We'll do that shortly, but first, let's try calling our function right in thescratch.ufile, just by starting a line with>:

use base

square : Nat -> Nat
square x = x * x

> square 4

And Unison replies:

6 | > square 4
⧩
16

That6 |is the line number from the file. The> square 4on line 6 of the file, starting with a>is called a "watch expression", and Unison uses these watch expressions instead of having a separate read-eval-print-loop (REPL). The code you are editing can be run interactively, right in the same spot as you are doing the editing, with a full text editor at your disposal, with the same definitions all in scope, without needing to switch to a separate tool.

Theuse baseis awildcard use clause.This lets us use anything from thebasenamespace under thelibnamespace unqualified. For example we refer tobase.Natas simplyNat.

Question:do we really want to reevaluate all watch expressions on every file save? What if they're expensive? Luckily, Unison keeps a cache of results for expressions it evaluates, keyed by the hash of the expression, and you can clear this cache at any time without ill effects. If a result for a hash is in the cache, Unison returns that instead of evaluating the expression again. So you can think of and use your.uscratch files a bit like spreadsheets, which only recompute the minimal amount when dependencies change.

🤓
There's one more ingredient that makes this work effectively, and that's functional programming. When an expression has no side effects, its result is deterministic, and you can cache it as long as you have a good key to use for the cache, like the Unison content-based hash. Unison's type system won't let you do I/O inside one of these watch expressions or anything else that would make the result change from one evaluation to the next.

Let's try out a few more examples:

-- A comment, ignored by Unison

> List.reverse [1,2,3,4]
> 4 + 6
> 5.0 / 2.0
> not true
✅

~/unisoncode/scratch.u changed.

Now evaluating any watch expressions (lines starting with
>)… Ctrl+C cancels.

6 | > List.reverse [1,2,3,4]
⧩
[4, 3, 2, 1]

7 | > 4 + 6
⧩
10

8 | > 5.0 / 2.0
⧩
2.5

9 | > not true
⧩
false

Let's add a test for oursquarefunction:

square : Nat -> Nat
square x = x * x

test> square.tests.ex1 =
use Nat ==
check (square 4 == 16)

Save the file, and Unison comes back with:

8 | test> square.tests.ex1 = check (square 4 == 16)

✅ Passed : Proved.

Some syntax notes:

• Thetest>prefix tells Unison that what follows is a test watch expression. Note that we're also giving a name to this expression,square.tests.ex1.
• Note: there's nothing special about the namesquare.tests.ex1;we could call those bindings anything we wanted. Here we use the convention that tests for a definitionfoogo infoo.tests.
• Note: these terms are being written while the UCM prompt is still reading.tour>,so adding or updating them would mean that the fully qualified name for, say,squarewould be.tour.square.

Thetest.checkfunction has the signaturetest.check : Boolean -> [test.Result].It takes aBooleanexpression and gives back a list of test results, of type[base.test.Result](tryview test.Result).In this case there was only one result, and it was a passed test.

### A property-based test

Let's test this a bit more thoroughly.squareshould have the property thatsquare a * square b == square (a * b)for all choices ofaandb.The testing library supports writing property-based tests like this. There's some new syntax here, explained afterwards:

use base

square : Nat -> Nat
square x = x * x

use test

test> square.tests.ex1 = check (square 4 == 16)

test> square.tests.prop1 =
go _ = a = !gen.natInOrder
b = !gen.natInOrder
expect (square a * square b == square (a * b))
runs 100 go
11 |   go _ = a = !natInOrder

✅ Passed : Passed 100 tests.

This will test our function with a bunch of different inputs.

#### Syntax notes

• The Unison block, which begins after an=,can have any number ofbindings(likea = …)all at the same indentation level, terminated by a single expression (hereexpect (square ..)),which is the result of the block.
• You can call a function parameter_if you just plan to ignore it. Here,goignores its argument; its purpose is just to makegolazily evaluatedso it can be run multiple times by therunsfunction.
• natInOrderis aA delayed computation is one in which the result is not computed right away. The signature for a delayed computation can be thought of as a function with no arguments, returning the eventual result:() -> a.The!in!gen.natInOrderevaluates the delayed computation, summoning a value of typeNat.
• natInOrdercomes fromtest-test.gen.natInOrder.It's a "generator" of natural numbers.!natInOrdergenerates one of these numbers starting at 0 and incrementing by one each time it is called.

## Adding code to the codebase

Thesquarefunction and the tests we've written for it are not yet part of the codebase. So far they only exist in our scratch file. Let's add them now. Switch to the Unison console and typeadd.You should get something like:

.tour> add

square             : Nat -> Nat
square.tests.ex1   : [Result]
square.tests.prop1 : [Result]

You've just added a new function and some tests to your Unison codebase under thetournamespace. Try typingview squareorview square.tests.prop1.Notice that Unison inserts preciseusestatements when rendering your code.usestatements aren't part of your code once it's in the codebase. When rendering code, a minimal set ofusestatements is inserted automatically by the code printer, so you don't have to be precise with yourusestatements.

If you typetestat the Unison prompt, it will "run" your test suite:

.tour> test

Cached test results (help testcache to learn more)

◉ square.tests.ex1      : Proved.
◉ square.tests.prop1    : Passed 100 tests.

✅ 2 test(s) passing

Tip: Use view square.tests.ex1 to view the source of a test.

But actually, it didn't need to run anything! All the tests had been run previously and cached according to their Unison hash. In a purely functional language like Unison, tests like these are deterministic and can be cached and never run again. No more running the same tests over and over again!

## Moving and renaming terms

When we addedsquare,we were at thetournamespace, sosquareand its tests are attour.square.We can also move the terms and namespaces to different locations in our codebase with themovecommands.

.tour> move.term .tour.square mySquare

Done.

.tour> find

1.  mySquare : base.Nat -> base.Nat

.tour> move.namespace square.tests mySquare.tests

Done.

We're using.tour.squareto refer to thesquaredefinition with an absolute path, and then moving it to therelativenamemySquare.When you're done shuffling some things around, you can usefindwith no arguments to view all the definitions under the current namespace:

.tour> find

1. mySquare : Nat -> Nat
2. mySquare.tests.ex1 : [Result]
3. mySquare.tests.prop1 : [Result]

Also notice that we don't need to rerun our tests after this reshuffling. The tests are still cached:

.tour> test

Cached test results (help testcache to learn more)

◉ mySquare.tests.ex1       : Proved.
◉ mySquare.tests.prop1     : Passed 100 tests.

✅ 2 test(s) passing

Tip:  Use view square.tests.ex1 to view the source of a test.

We get this for free because the test cache is keyed by the hash of the test, not by what the test is called.

When you're starting out writing some code, it can be nice to just put it in a temporary namespace, perhaps calledtemporscratch.Later, without breaking anything, you can move that namespace or bits and pieces of it elsewhere, using theandmove.namespacecommands.

## Modifying existing definitions

Instead of starting a function from scratch, often you just want to slightly modify something that already exists. Here we'll make a change to the implementation of ourmySquarefunction.

### Using theeditcommand

Try doingedit mySquarefrom your prompt (note you can use tab completion):

.tour> edit mySquare
☝️

I added these definitions to the top of ~/unisoncode/scratch.u

mySquare : Nat -> Nat
mySquare x =
use Nat *
x * x

You can edit them there, then do update to replace the definitions currently in this branch.

This copies the pretty-printed definition ofmySquareinto your scratch file "above the fold." That is, it adds a line starting with---and puts whatever was already in the file below this line. Unison ignores any file contents below the fold.

Let's editmySquareand instead definemySquare x(just for fun) as the sum of the firstxodd numbers (here's a

use base

mySquare : Nat -> Nat
mySquare x =
sum (map (x -> x * 2 + 1) (range 0 x))

sum : [Nat] -> Nat
sum = foldLeft (+) 0
✅

I found and typechecked these definitions in ~/unisoncode/scratch.u. If you do an

⍟ These new definitions are ok to add:

sum : [Nat] -> Nat

⍟ These names already exist. You can update them to your new definition:

mySquare : Nat -> Nat

### Adding an updated definition to the codebase

Notice the message says thatmySquareis ok toLet's try that:

.tour> update

sum : [Nat] -> Nat

⍟ I've updated these names to your new definition:

mySquare : Nat -> Nat

### Only affected tests are rerun onupdate

If we rerun the tests, the tests won't be cached this time, since one of their dependencies has actually changed:

.tour> test

✅

New test results:

◉ mySquare.tests.ex1      : Proved.
◉ mySquare.tests.prop1    : Passed 100 tests.

✅ 2 test(s) passing

Tip: Use view mySquare.tests.ex1 to view the source of a test.

Notice the message indicates that the tests weren't cached. If we dotestagain, we'll get the newly cached results.

The dependency tracking for determining whether a test needs rerunning is 100% accurate and is tracked at the level of individual definitions. You'll only rerun a test if one of the individual definitions it depends on has changed.

## Publishing code and installing Unison libraries

🎉
The last you'll need to do to get set up to write Unison code is to sign up for Unison-share! Head toUnison Shareand follow the instructions there to link your local codebase for code hosting!

Code is published to Unison's own code hosting solution,using thepushcommand and libraries are installed via thepullcommand. There's no separate tooling needed for managing dependencies or publishing code, and you'll never encounter dependency conflicts in Unison.