hot-language

name	hot-language
description	Write code in the Hot programming language. Use when creating, editing, or reviewing .hot files. Hot is a functional, expression-based language with automatic parallelization, no infix operators, and expression-based assignment syntax. IMPORTANT: Hot syntax differs significantly from conventional languages - always check these references before writing Hot code.
metadata	{"author":"hotdev","version":"1.0","license":"Apache-2.0"}

Hot Language Skill

Hot is a functional, expression-based language with automatic parallelization and type inference. This skill provides detailed reference documentation for the Hot programming language.

Note: For quick syntax rules, see AGENTS.md in the project root. This skill contains extended reference documentation.

Critical Syntax Rules

These are non-negotiable - violating them causes parse errors:

❌ Wrong	✅ Correct	Rule
name = "Alice"	name "Alice"	No = for assignment
a + b	add(a, b)	No infix operators
a == b	eq(a, b)	Comparison is a function
if (x) { } else { }	if(x, then, else) or cond flow	No if/else blocks
for x in items	map(items, ...) or for-each(iter, ...)	No loops
items.0	items[0]	Use brackets for array indexing
add (1, 2)	add(1, 2)	No space before ( when calling
{x} for a map	{x,} or {x: x}	Single-key punning needs comma

File Structure

Every Hot file must start with a namespace declaration:

::myapp::users ns

// Namespace aliasing: create short aliases
::http ::hot::http
::env ::hot::env

// Import specific items
HttpResponse ::hot::http/HttpResponse

// Variables (no = sign)
api-url ::env/get("API_URL", "https://api.example.com")
max-retries: Int 3

// Functions (return type is the success type, not Result)
get-user fn (id: Str): Map {
    response ::http/get(`${api-url}/users/${id}`)
    if(is-ok(response), ok(response.body), err("Failed"))
}

Quick Reference

Variables

name "Alice"              // Inferred type
count: Int 42             // Explicit type
config {key: "value"}     // Map literal
query """                 // Block string (indent-aware, no interpolation)
    SELECT * FROM users
    WHERE active = true
    """
tpl ```                   // Block template string (indent-aware + interpolation)
    SELECT * FROM ${table}
    WHERE id = ${id}
    ```

Deep Paths

user.name "Alice"
user.settings.theme "dark"
servers[0] "api.example.com"
items[] "first"           // append to vector
shopping.items[] "apple"  // append to nested vector

Functions

// Basic function
greet fn (name: Str): Str {
    `Hello, ${name}!`
}

// Conditional flow function
classify fn cond (x: Int): Str {
    lt(x, 0) => { "negative" }
    eq(x, 0) => { "zero" }
    => { "positive" }  // Default case
}

// Parallel execution function
fetch-all fn parallel (id: Str): All<Map> {
    user ::http/get(`/users/${id}`)
    posts ::http/get(`/users/${id}/posts`)
}

Flows

Flows are standalone constructs. The fn keyword makes them function definitions.

• serial - Sequential execution (default)
• cond - First matching branch wins
• cond-all - ALL matching branches execute, returns Map
• match - Pattern match on types/values, first match wins; arms support unions (A | B), T? sugar for T | Null, and Any (matches everything)
• match-all - Pattern match, ALL matches execute
• parallel - Concurrent execution, returns Map

process fn (data: Map): Map {
    // Standalone cond flow (errors propagate automatically)
    validated cond {
        is-empty(data) => { err("Empty") }
        => { data }
    }

    // Parallel flow
    results parallel {
        fetch-a(validated)
        fetch-b(validated)
    }

    results
}

Types

// Struct type
User type { id: Str, email: Str, active: Bool }
user User({id: "1", email: "[e-mail]", active: true})

// Enum type — `match` must cover every variant or use a `_` default arm
Direction enum { Up, Down, Left, Right }
dir Direction.Up

// Enum with data
Shape enum { Circle(Circle), Point }

// Open enum — extensible via `Source -> Enum.Variant` arrows.
// Match on an open enum REQUIRES a `_` default arm.
Animal enum open { Dog, Cat }
Bird type { species: Str }
Bird -> Animal.Bird               // bodyless arrow enrolls the variant

// Literal union (enum-like, raw values)
Fruit type "apple" | "banana" | "orange"

// Open literal union — extensible by re-declaring at top level.
// Leading `|` makes single-member extensions read naturally.
HttpMethod type open "GET" | "POST" | "PUT" | "DELETE" | "PATCH"
HttpMethod type open | "CONNECT"
HttpMethod type open | "TRACE"

// Type coercion
Date -> Str fn (d: Date): Str { `${d.year}-${d.month}-${d.day}` }

Common Functions

• Math: add, sub, mul, div, mod, abs, round
• Comparison: eq, ne, lt, gt, lte, gte
• Logic: and, or, not, if
• Collections: map, filter, reduce, first, rest, concat, length, get
• Strings: uppercase, lowercase, trim, split, join
• Results: ok, err, is-ok, is-err

Pipe Operator

Piped value becomes the first argument:

result 5 |> add(2) |> mul(3)  // add(5,2)=7, mul(7,3)=21

App and Platform Patterns

// Function aliases can carry metadata and point at another function.
signup-webhook
meta {webhook: {service: "leads", path: "/signup"}}
handle-signup

// Use HttpRequest for headers/raw body; simple get/post helpers have fixed arity.
HttpRequest ::hot::http/HttpRequest
response ::hot::http/request(HttpRequest({
    method: "POST",
    url: "https://api.example.com/users",
    headers: {Authorization: `Bearer ${api-key}`},
    body: untype(user),
}))

Event Handlers and Schedules

// Event handler
on-user-created meta {on-event: "user:created"}
fn (event) { send-email(event.data.email) }

// With retry
on-payment meta {on-event: "payment:received", retry: 3}
fn (event) { process(event.data) }

// Scheduled function
daily-sync meta {schedule: "0 2 * * *", retry: {attempts: 5, delay: 10000}}
fn () { sync-data() }

Testing

test-add meta ["test"]
fn () {
    assert(eq(add(1, 2), 3), "1 + 2 should equal 3")
}

Run tests with hot test.

Reference Documentation

This skill includes detailed reference files:

File	Description
references/syntax.md	Complete syntax reference
references/hot-std.md	Full standard library documentation
references/flows.md	Flow patterns, if() vs cond, flow result shapes
references/types.md	Type system, type coercion
references/error-handling.md	Auto-unwrapping, lazy arguments, Result patterns
references/sequences.md	Eager collections, lazy iterators, the Next type
references/meta.md	Metadata for tests, events, schedules, MCP tools, webhooks, and context

Code Examples

See the examples/ directory for working Hot code:

• basic.hot - Variables, functions, conditionals, pipes
• types-and-enums.hot - Custom types, enums (closed + open), literal unions (closed + open), arrow enrollment, exhaustive match, type coercion
• flows.hot - Parallel, cond-all, match-all, flow result shapes
• error-handling.hot - Auto-unwrapping, lazy arguments, Result patterns
• sequences.hot - Eager collections, lazy iterators, range functions
• event-handlers.hot - Events, schedules, retry patterns
• http-service.hot - HTTP requests, API integration
• webhooks.hot - Inbound webhooks, function aliases, and HttpResponse
• stores.hot - Persistent stores, list entry shape, and semantic search setup

CLI Commands

hot dev             # Start dev server (watches for changes)
hot run file.hot    # Run a Hot file
hot check           # Type check the project
hot test            # Run tests
hot repl            # Interactive REPL
hot deploy          # Deploy to Hot cloud

See Also

• AGENTS.md in project root - Quick syntax reference (passive context)
• hot.dev/docs - Full online documentation