Tooling

VS Code Extension

The vscode-seen/ directory contains a full-featured VS Code extension.

Installation

cd vscode-seen
npm install
npm run package
code --install-extension seen-*.vsix

Features

• Syntax highlighting -- TextMate grammar covering keywords, types, operators, strings, comments
• IntelliSense -- via built-in LSP (completions, hover, go-to-definition)
• Real-time diagnostics -- error and warning reporting
• Code formatting -- Shift+Alt+F formats the current file
• Debugging -- debug adapter for Seen programs
• 34 code snippets -- common patterns (see below)

Commands

Command	Shortcut	Description
Seen: Build	Ctrl+Shift+B	Build project
Seen: Run	F5	Run project
Seen: Format	Shift+Alt+F	Format document
Seen: Check	--	Type check project
Seen: Test	--	Run tests
Seen: Clean	--	Clean build artifacts
Seen: Init	--	Initialize new project
Seen: REPL	--	Open interactive REPL
Seen: Switch Language	--	Change keyword language

Configuration

{
    "seen.compiler.path": "seen",
    "seen.lsp.enabled": true,
    "seen.lsp.trace.server": "off",
    "seen.formatting.enable": true,
    "seen.target.default": "native",
    "seen.language.default": "en"
}

Code Snippets

Prefix	Description
main	Main function entry point
fun	Function declaration
class	Class with constructor
struct	Struct declaration
enum	Enum declaration
trait	Trait declaration
impl	Trait implementation
if / ife	If / if-else
for	For-in loop
while	While loop
when / match	Pattern matching
let / var	Variable declarations
async	Async function
compute	GPU compute shader
parallel_for	Parallel for loop
simd	SIMD vector construction
trycatch	Try-catch block
defer / errdefer	Deferred cleanup
extern	External/FFI function
derive	Derive macro
test / bench	Test/benchmark function
unsafe	Unsafe block
region / arena	Memory region/arena
println	Print with newline

Language Server Protocol (LSP)

Built-in LSP

seen lsp

Features:

• Diagnostics (errors and warnings)
• Completions
• Hover information
• Go-to-definition
• Find references
• Semantic tokens

Neovim

require'lspconfig'.seen.setup{
  cmd = {"seen", "lsp"},
  filetypes = {"seen"},
  root_dir = require'lspconfig.util'.root_pattern("Seen.toml", ".git"),
}

Emacs

(lsp-register-client
 (make-lsp-client :new-connection (lsp-stdio-connection '("seen" "lsp"))
                  :major-modes '(seen-mode)
                  :server-id 'seen-lsp))

Code Formatting

seen fmt <source.seen>

Formatting rules:

• 4-space indentation
• Trim trailing whitespace
• Ensure final newline
• Configurable via [format] in Seen.toml:

[format]
line-width = 100
indent = 4
trailing-comma = true

Testing

seen test

Write tests with the @test decorator:

@test
fun test_addition() {
    assert(1 + 1 == 2, "basic addition")
}

@test
fun test_string_concat() {
    let result = "hello" + " " + "world"
    assert(result == "hello world", "string concat")
}

Run end-to-end tests across all languages:

bash tests/e2e_multilang/run_all_e2e.sh

Debugging

Environment variable tracing

# Type checker
SEEN_DEBUG_TYPES=1 seen build program.seen

# LLVM IR generation
SEEN_TRACE_LLVM=all seen build program.seen

# Struct layout
SEEN_TRACE_LLVM=gep seen build program.seen

LLVM IR inspection

seen build program.seen --emit-llvm
cat program.ll

Debug symbols

seen build program.seen -g -o program
gdb ./program

Compile database

seen build program.seen --emit-compile-db
# produces compile_commands.json

Linux ARM64 Cross Sysroot

On pacman-compatible Linux hosts, you can create a local AArch64 cross sysroot for Seen without installing system packages globally:

./scripts/setup_linux_arm64_sysroot.sh
source artifacts/toolchains/linux-arm64/env.sh

The helper downloads and extracts the Arch/CachyOS cross-packages under artifacts/toolchains/linux-arm64/. The generated env.sh sets SEEN_LINUX_ARM64_SYSROOT and SEEN_LINUX_ARM64_GCC_TOOLCHAIN, allowing compiler_seen/target/seen and the native smoke harness to produce Linux ARM64 binaries on x86_64 Linux hosts.

Validate the local setup with either command:

bash scripts/native_target_smoke.sh --compiler compiler_seen/target/seen --target linux-arm64
bash scripts/platform_matrix.sh --stage3 compiler_seen/target/seen --platform linux-arm64

For hosts that can execute the produced binaries directly, platform_matrix.sh --with-runtime also runs lightweight runtime checks in addition to compile smoke. On Apple Silicon macOS, the Apple runtime subset now covers hello_english, test_comptime_target_predicates, hash_map_basic, string_hash_map_basic, str_basic, and string_buffer_basic for macos-arm64, macos-x86_64, and ios-sim-arm64.

The same command also exposes a capability-gated ios-arm64 physical-device lane. Pass --ios-device <udid> to target a specific iPhone, --ios-provisioning-profile <path> to embed a development profile, and optionally --ios-sign-identity, --ios-entitlements, and --ios-bundle-prefix if your local signing setup needs them. When no provisioned device is available, the report shows unavailable instead of reporting a successful lane.

If you want the helper to replace an existing extracted toolchain directory, rerun it with --force.

Import from C

Generate Seen bindings from a C header:

seen import-c mylib.h

Outputs extern fun declarations that can be pasted into Seen source.

Related

• Getting Started -- editor setup
• CLI Reference -- all flags
• Project Configuration -- Seen.toml format