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:

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
      ⧩
      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 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 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

Unison's interactive scratch files

Deterministic