Playground & WASM

When running in the browser playground at sema.run, Sema executes as WebAssembly. Most stdlib functions work identically, but some behave differently due to browser sandbox constraints, and a few web-only functions are available.

Chromium ARM64 Note

Chrome/Chromium builds before version 147 contain a V8 ARM64 WebAssembly compiler bug that can crash the renderer on some heavy tree-walker workloads in the playground. If you hit a reproducible tab crash on Apple Silicon, update Chrome or retry in Firefox, Safari, or Chrome 147+.

Web-Only Functions

These functions are only available in the WASM playground — they access browser APIs that don't exist in the native CLI.

web/user-agent

Return the browser's navigator.userAgent string. Works in all browsers.

(web/user-agent)
; => "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 ..."

web/user-agent-data

Return structured browser information from navigator.userAgentData. Returns a map on Chromium-based browsers (Chrome, Edge, Opera), nil on Firefox and Safari.

(web/user-agent-data)
; Chromium => {:mobile false :platform "macOS" :brands ("Chromium/120" "Google Chrome/120")}
; Firefox/Safari => nil

TIP

userAgentData is the modern replacement for UA string parsing — it returns structured, reliable data instead of a messy string. However, it's Chromium-only. Use web/user-agent for cross-browser compatibility.

WASM Behavior Differences

System Information

System functions return web-appropriate values instead of OS-specific ones:

Function	Native	WASM
sys/platform	"macos" / "linux" / "windows"	"web"
sys/os	"macos"	"web"
sys/arch	"aarch64" / "x86_64"	"wasm32"
sys/cwd	Current directory path	"/"
sys/interactive?	#t in REPL	#f
sys/pid	Process ID	0
sys/elapsed	Nanoseconds since process start	Nanoseconds since page load
time-ms	SystemTime milliseconds	Date.now() milliseconds

These always return nil in WASM: sys/hostname, sys/user, sys/home-dir, sys/which, sys/tty.

File I/O (Virtual Filesystem)

File operations work against an in-memory virtual filesystem (VFS). Files persist for the duration of your session but are lost on page reload.

;; These all work in the playground
(file/write "/hello.txt" "Hello from WASM!")
(file/read "/hello.txt")       ; => "Hello from WASM!"
(file/exists? "/hello.txt")    ; => #t
(file/mkdir "/mydir")
(file/is-directory? "/mydir")  ; => #t
(file/list "/")                ; => ("hello.txt")

All file functions are supported: file/read, file/write, file/append, file/delete, file/rename, file/copy, file/exists?, file/list, file/mkdir, file/is-file?, file/is-directory?, file/is-symlink?, file/info, file/read-lines, file/write-lines. Path functions (path/join, path/dirname, path/basename, path/extension, path/absolute) also work.

Quotas: The VFS enforces limits to prevent runaway memory usage — 1 MB per file, 16 MB total, and 256 files max. Exceeding these limits returns an error.

The load function reads from the VFS and evaluates the parsed expressions.

Terminal Styling

All term/* functions work but return text without ANSI formatting (since the browser has no terminal):

(term/bold "hello")   ; => "hello" (no bold applied)
(term/red "error")    ; => "error" (no color applied)
(term/style "hi" :bold :cyan)  ; => "hi"

HTTP Functions

HTTP functions work in the playground via the browser's fetch() API. They return the same {:status :headers :body} map as the native CLI.

(define resp (http/get "https://httpbin.org/get"))
(:status resp)    ; => 200
(:body resp)      ; => "{\"args\": {}, ...}"

(http/post "https://httpbin.org/post" {:name "sema"})
; => {:status 200 :headers {...} :body "..."}

All HTTP functions are supported: http/get, http/post, http/put, http/delete, http/request.

CORS Restrictions

Browser security rules (CORS) may block requests to servers that don't include Access-Control-Allow-Origin headers. Public APIs like httpbin.org work fine. If you get a network error, the target server likely doesn't allow cross-origin requests.

Not Available in WASM

These functions return an error when called in the playground:

Function	Reason
shell	No subprocess execution in browser
exit	No process to exit
io/read-line	No stdin in browser
io/read-stdin	No stdin in browser
sleep	Cannot block the browser main thread (no-op)