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Build your first Mochi program

Build a command-line app that tracks a personal book reading list. The program adds books, marks them as read, queries them, and persists state to JSON. The language gets introduced step by step alongside the code.

You will:

  • Organize a multi-file Mochi project as a package.
  • Define custom types with inline methods.
  • Co-locate test blocks with the code they cover.
  • Drive the program from command-line arguments.
  • Persist state to a JSON file.
  • Query records with from / where / select.

If Mochi is not installed yet, start with the quickstart. For a single-page summary of every piece of syntax, see language basics.

Source code

The finished program lives at examples/tutorials/reading-list/ in the main repo. Skip ahead to read it whole.

1. Set up the project​

Create a directory and a single empty file. Mochi has no project metadata file. A directory with .mochi files in it is already a package.

mkdir -p reading-list
cd reading-list
touch main.mochi

Open main.mochi and add a single line so the program runs end to end:

main.mochi
print("reading-list v0.1")

Run it:

mochi run main.mochi
reading-list v0.1

Mochi runs top-level statements in file order. No main function required.

2. Variables and the binding rules​

Add a couple of bindings at the top of the file:

main.mochi
let app_name = "reading-list"
let app_version = "0.1.0"
var run_count: int = 0

run_count = run_count + 1
print(app_name, "v" + app_version, "run #" + str(run_count))

let is immutable. Reassignment is a compile error. var is mutable. Annotations are optional; the compiler infers types when omitted. The run_count: int annotation is explicit, and str() converts the integer for concatenation.

mochi run main.mochi
reading-list v0.1.0 run #1
Why str()?

Mochi does not implicitly convert numbers to strings. + on a string and an int is a type error, which catches a class of bugs at compile time. The errors page goes deeper.

3. The Book type​

The app needs a Book record. Declare it with type:

type Book {
  title: string
  author: string
  pages: int
  read: bool
}

Construct values with brace syntax. Field order matches the declaration.

let mythical = Book {
  title: "The Mythical Man-Month",
  author: "Frederick Brooks",
  pages: 322,
  read: true
}

print(mythical.title, "by", mythical.author)

Inline methods reference fields directly. There is no explicit self parameter.

type Book {
  title: string
  author: string
  pages: int
  read: bool

  fun summary(): string {
    let status = if read { "[x]" } else { "[ ]" }
    return status + " " + title + " by " + author + " (" + str(pages) + "p)"
  }
}

print(mythical.summary())
[x] The Mythical Man-Month by Frederick Brooks (322p)

4. A list of books​

Lists use [ ] literal syntax. The element type is inferred from the contents.

var library: list<Book> = [
  Book {
    title: "The Mythical Man-Month",
    author: "Frederick Brooks",
    pages: 322,
    read: true
  },
  Book {
    title: "Designing Data-Intensive Applications",
    author: "Martin Kleppmann",
    pages: 616,
    read: false
  }
]

for book in library {
  print(book.summary())
}

A var list is mutable in place (library.push(...) works) and reassignable to a different list.

5. Functions and pure helpers​

A helper to add a book:

fun add_book(books: list<Book>, b: Book): list<Book> {
  return books + [b]
}

+ on two lists concatenates them. The function returns a new list rather than mutating the input. Pure functions are easier to reason about, easier to test, and easier to memoize.

A second helper marks a title as read:

fun mark_read(books: list<Book>, title: string): list<Book> {
  return books | map(fun(b: Book): Book => {
    if b.title == title {
      return Book { title: b.title, author: b.author, pages: b.pages, read: true }
    }
    return b
  })
}

| is the pipeline operator: value | f is f(value). map is in the prelude. The lambda updates the matching book without touching the others.

Try it:

let updated = mark_read(library, "Designing Data-Intensive Applications")
for b in updated {
  print(b.summary())
}
[x] The Mythical Man-Month by Frederick Brooks (322p)
[x] Designing Data-Intensive Applications by Martin Kleppmann (616p)

6. Tests live with the code​

Co-locate tests with the functions they cover. Each test block has a quoted name and a list of expect assertions.

test "add_book appends" {
  let before = []
  let after = add_book(before, Book {
    title: "x", author: "y", pages: 1, read: false
  })
  expect len(after) == 1
  expect after[0].title == "x"
}

test "mark_read flips the flag" {
  let books = [
    Book { title: "a", author: "x", pages: 1, read: false },
    Book { title: "b", author: "y", pages: 2, read: false }
  ]
  let updated = mark_read(books, "b")
  expect updated[0].read == false
  expect updated[1].read == true
}

Run the tests:

mochi test main.mochi
2 tests passed

A failing expect reports the line, the failing expression, and the left- and right-hand values. No setup, no fixtures, no separate runner.

7. Querying with from / where / select​

How many unread books are in the library? A for loop works, but Mochi has a query expression that reads more like SQL.

let unread = from b in library
             where !b.read
             sort by b.pages
             select b

print("you have", len(unread), "unread book(s):")
for b in unread {
  print("  -", b.title, "(" + str(b.pages) + "p)")
}

The same query rewrites as filter / sort_by / map, and sometimes that is the right call. For record-shaped data the query form is hard to beat. Read more in datasets.

8. Reading and writing JSON​

The library is hard-coded so far. Replace the literal with a load from disk and a save back.

Create library.json:

library.json
[
  { "title": "The Mythical Man-Month",
    "author": "Frederick Brooks",
    "pages": 322,
    "read": true },
  { "title": "Designing Data-Intensive Applications",
    "author": "Martin Kleppmann",
    "pages": 616,
    "read": false }
]

Load it:

let library = load "library.json" as Book

load parses the file based on its extension and decodes each record into the named type. CSV, JSON, JSONL, and YAML are supported.

save is the inverse:

let updated = mark_read(library, "Designing Data-Intensive Applications")
save updated to "library.json"

The file is rewritten in JSON. For a different format, add as csv, as jsonl, or as yaml to the save clause.

9. Splitting into a package​

main.mochi is getting busy. Move the Book type and helpers into a sibling file.

reading-list/book.mochi
package reading_list

export type Book {
  title: string
  author: string
  pages: int
  read: bool

  fun summary(): string {
    let status = if read { "[x]" } else { "[ ]" }
    return status + " " + title + " by " + author + " (" + str(pages) + "p)"
  }
}

export fun add_book(books: list<Book>, b: Book): list<Book> {
  return books + [b]
}

export fun mark_read(books: list<Book>, title: string): list<Book> {
  return books | map(fun(b: Book): Book => {
    if b.title == title {
      return Book { title: b.title, author: b.author, pages: b.pages, read: true }
    }
    return b
  })
}

Names visible outside the package are marked with export. Names without export are package-private.

main.mochi imports the package:

main.mochi
import "./reading-list" as rl

let library = load "library.json" as rl.Book
for b in library {
  print(b.summary())
}

Local imports start with ./ or ../ and resolve relative to the importing file. The package alias is the last path segment unless overridden with as. Read more in packages.

10. A CLI entry point​

A real reading-list app takes commands from the user. Mochi exposes the program arguments as args:

main.mochi
import "./reading-list" as rl

let library = load "library.json" as rl.Book

let cmd = if len(args) > 0 then args[0] else "list"

match cmd {
  "list" => {
    for b in library {
      print(b.summary())
    }
  },
  "unread" => {
    let unread = from b in library where !b.read select b
    for b in unread {
      print(b.summary())
    }
  },
  "read" => {
    let title = args[1]
    let updated = rl.mark_read(library, title)
    save updated to "library.json"
    print("marked", title, "as read")
  },
  "add" => {
    let b = rl.Book {
      title: args[1],
      author: args[2],
      pages: to_int(args[3]),
      read: false
    }
    let updated = rl.add_book(library, b)
    save updated to "library.json"
    print("added", b.title)
  },
  _ => print("usage: reading-list [list | unread | add | read]")
}

Run a few commands:

mochi run main.mochi list
mochi run main.mochi unread
mochi run main.mochi add "Crafting Interpreters" "Robert Nystrom" 600
mochi run main.mochi read "Crafting Interpreters"
[x] The Mythical Man-Month by Frederick Brooks (322p)
[x] Designing Data-Intensive Applications by Martin Kleppmann (616p)
added Crafting Interpreters
marked Crafting Interpreters as read

to_int is in the prelude. It panics on invalid input. See errors for the safer safe_to_int form that returns int | nil.

11. Packaging as a single binary​

Once the program works, build it into a self-contained binary:

mochi build main.mochi -o reading-list
./reading-list list

The output binary embeds the Mochi runtime. It runs anywhere the host OS runs without a separate Mochi install on the target machine.

To stay scripted, mochi run caches compiled bytecode under ~/.cache/mochi, so subsequent runs approach binary speed.

Next​

  • HTTP fetch shows how to query an open book API from this project.
  • Generative AI drives model calls. Use generate text { ... } to recommend a next book based on the read pile.
  • Agents wraps the library in an agent that emits a BookAdded event whenever a new title arrives.
  • mochi transpile main.mochi --to go produces an equivalent Go program; same with --to python and --to typescript.
  • Language basics is a single-page tour of the syntax.
  • Reference is the concept-by-concept index.