🗺 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 documentationis 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 throughthe big idea.

Part 1: 👋 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. Put differently, the UCM is the interface to your Unison codebase.

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

💡 Remember: Unison code is not saved as text-based file content. That's why we need a tool that lets us change and run Unison programs.

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)...


Get started:

  📖 Type help to list all commands, or help <cmd> to view help for one command
  🎨 Type ui to open the Codebase UI in your default browser
  📚 Read the official docs at https://www.unison-lang.org/docs/
  🌏 Visit Unison Share at https://share.unison-lang.org to discover libraries
  👀 I'm watching for changes to .u files under ~/unisonCode


What's happening here? This is the Unison Codebase Manager starting up and initializing a fresh codebase. 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 saved and updated programatically, with the aid of this CLI.

The Unison codebase format has a few key properties:

  • Terms and types are identified by their implementation, not just their name. The codebase stores a hash of the syntax tree of a term or type.
  • It isappend-only:all changes to the codebase, including actions like deleting a term or deleting a branch, are appended to a log representing the codebase state.
  • As a result, a Unison codebase has its own notion of versioning and synchronization independent of git, anda robust set of toolshas been developed for managing and viewing changes in Unison code over time.

The codebase location the UCM drops you in if you have never used it before is represented by the.in the prompt,.>.We call this "the root" of your codebase, but most of the time your prompt will be prefixed with the name of the most recent "project" you've worked on.

Part 2: 🗂 Creating a project

Rather than creating multiple codebases for each application you're working on, Unison subdivides the codebase into "projects". Unisonprojectsare analogous to source code repositories. They help organize your codebase into applications, libraries, and other work that you may want to collaborate with others on. Projects are further divided intobranchesfor representing independent work streams. We'll introduce the concept of projects by creating one for thistourand establishing some conventions for organizing it.

Inside a project, your code is further organized into Unisonnamespaces.Namespaces are mappings from human-readable names to their definitions. Names in Unison are things like:math.sqrt,base.Optional.Some,base.Nat,base.Nat.*,++,orfoo.That is one or more segments separated by a.,with the last segment allowed to be an operator name like*or++.

We often think of these name segments as forming a tree, much like a directory of files. You'll be working with your prompt at the root of your project most of the time in Unison.

In the codebase manager console, create atourproject with theproject.createcommand. This is where you'll be adding code for the remainder of this guide.

.> project.create tour

   🎉 I've created the project tour.

   I'll now fetch the latest version of the base Unison library...

   🎨 Type `ui` to explore this project's code in your browser.
   🌏 Discover libraries at https://share.unison-lang.org
   📖 Use `help-topic projects` to learn more about projects.

   Write your first Unison code with UCM:

     1. Open scratch.u.
     2. Write some Unison code and save the file.
     3. In UCM, type `add` to save it to your new project.

   🎉 🥳 Happy coding!


Notice the prompt changes totour/main>,indicating your current project is nowtourand your current branch is/main.When editing Unison code, and interacting with the UCM your UCM commands and code are "scoped" to this project and branch.

When you create a new project, the UCM automatically installs thebasestandard library for you. It's located in a special namespace calledlib.


Unison looks for project dependencies directly under theproject root.Thelibdirectory in ourtourproject will contain all the dependencies necessary for running the code in the project.

If youcdinto a namespace which does not have thelibdirectory inside of it, the UCM will not be able to find the dependencies for your project.

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

Part 3: ⛵️ Navigating a Unison codebase

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

tour/main> ls lib.base.data.List

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

tour/main> ls lib.base.data.List

1.   ++                    ([a] -> [a] -> [a])
2.   +:                    (a -> [a] -> [a])
3.   :+                    ([a] -> a -> [a])
4.   Nonempty              (type)

Because of the 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, 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 twofindcommands (new syntax is explained below):

tour/main> find reverse

1.  lib.base.data.List.Nonempty.reverse : List.Nonempty a -> List.Nonempty a
2.  lib.base.data.List.reverse : [a] -> [a]
3.  lib.base.Text.reverse : Text -> Text
4.  lib.base.data.List.Nonempty.reverse.doc : Doc

Thefindcommand here is searching for definitions whose names includereverse.It searches first within our own code in the project, and then in the dependencies inlib.

tour/main> find : [a] -> [a]

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

tour/main> view 6

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

Here, we did a type-based search, 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:

  • lib.base.data.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[]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 beuniversally quantified,so[a] -> [a]also meansforall a . [a] -> [a].This type signature is saying that this is a function which takes a list of elements of some type and returns a list of elements of the same type.
  • List.reverse astakes one parameter, calledas.The stuff after the=is called thebodyof the function, and here it's ablock,which 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 anyoperator,sof x y + g p qparses as(f x y) + (g p q).You can always use parentheses to control grouping more explicitly.
Enter theviewcommand without any arguments to open up afzf searchinterface for finding definitions in your codebase. It's a quick way to explore the codebase and find definitions by name.

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

The Unison codebase, in its definition forlib.base.data.List.reverse,doesn't store names for the definitions it depends on (like thelib.base.data.List.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.lib.base.data.List.reverseis defined usinglib.base.data.List.foldLeft.Let's rename that toList.foldltemporarily. Try out the following command (you can use tab completion here if you like):

tour/main> move lib.base.data.List.foldLeft lib.base.data.List.foldl


tour/main> view lib.base.data.List.reverse

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

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. The two names are, to Unison, the same thing.

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 so choose! Naming things is hard enough, renaming them shouldn't be a trial.

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 lib.base.data.List.foldLeft lib.base.data.List.foldl
2. #na6fel77ai : project.create tour
3. #sjg2v58vn2 : (initial reflogged namespace)

The reflog keeps track of the history of the codebase by recording the hash of the rootnamespaceof your entire codebase. Namespace hashes change along with updates to the term and type definitions that they enclose. When we renamedlib.base.data.List.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 themovecommand was issued.

tour/main> reset-root #na6fel77ai


Great! OK, go drink some water, 🚰 and then let's start writing some Unison code!

Part 4: 📝 Writing Unison code

Unison's interactive scratch files

The codebase manager 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.

Now put the following in your scratch file:

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

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

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 : Nat -> Nat

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

It typechecked thesquarefunction and tells us thatsquareis "ok toadd."We'll do that shortly, but first, let's try calling our function right in thescratch.ufile, just by starting a line with>:

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

> square 4

And Unison prints:

6 | > square 4

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

Theuse Nat *is ause clausewhich specifies which "*" operator we want to use. This one is from theNatnamespace in ourlib.basestandard library. Use clauses mean we can refer tobase.Natas simplyNatand can refer to*without prefixing itNat.*.

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 isdeterministic,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 morewatch expressions:

-- 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

  8 | > 5.0 / 2.0

  9 | > not true

Testing your code

Next let's add a test for oursquarefunction:

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

test> square.tests.ex1 =
 verify '(ensureEqual (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.
  • 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.

Theverifyfunction has the signatureverify : '{g, Exception, Each, Random} a ->{g} [test.Result].It handles theEach,RandomandExceptionabilities. These are commonabilitiesused in testing. TheensureEqualfunction raises anExceptionif the two values are not equal, failing the test. In this case, the two values were equal, so the test passes.

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 = verify '(ensureEqual (square 4) 16)

test> square.tests.prop1 =
   verify do
     Each.repeat 100
     a = Random.natIn 0 100000
     b = Random.natIn 0 100000
     ensure (square a * square b == square (a * b))
10 |               verify do

✅ Passed

This will test our function with a bunch of different inputs using theEachandRandomabilities.Eachis being used to generate 100Randomnumbers, which the the test then verifies the property holds for.

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 (hereensure (square ..)),which is the result of the block.
  • The both the single quote'syntax and thedokeyword are ways of introducing adelayed computation.A 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.

Part 5: ➕➖ Adding and updating code

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 UCM and typeadd.You should get something like:

tour/main> add

  ⍟ I've added these definitions:

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

Try typingview squareorview square.tests.prop1.Notice that Unison inserts preciseusestatements when rendering your code. A minimal set ofusestatements is inserted automatically by the code printer upon viewing or editing definitions.

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

tour/main> test

  Cached test results (`help testcache` to learn more)

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

  ✅ 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 won't actually rerun until the code they depend on changes!

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/main> move square mySquare


tour/main> find

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

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


When you're done shuffling some things around, you can usefindwith no arguments to view all the definitions under the current namespace:

tour/main> 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.

tour/main> test

  Cached test results (`help testcache` to learn more)

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

  ✅ 2 test(s) passing

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

The tests are still cached because the test cache is keyed by the hash of the test itself, not by what the test happens to be called.

When you're starting out writing some code, sometimes it's nice to 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 themovecommand.

Updating existing definitions

Here we'll make a change to the implementation of ourmySquarefunction.

Try enteringedit mySquarein the UCM:

tour/main> 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 anice geometric illustration of why this gives the same results):

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
''add'' or ''update'' , here's how your codebase would change:

    ⍟ 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 toupdate.Let's try that:

tour/main> update

  ⍟ I've added these definitions:

    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/main> test


    New test results:

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

  ✅ 2 test(s) passing

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

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.

Part 6: 🤝 Sharing 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! Unison Share is the place to publish and discover Unison libraries. 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,Unison Share,using thepushcommand and libraries are installed via thepullcommand. There's no separate tooling needed for managing dependencies or publishing code. It's all built into the UCM.

Congratulations on completing the tour of Unison! You're ready to get writing Unison code. We're excited to see what you build! 🥳