Update Cookbook.md

docs/Cookbook.md

@@ -150,6 +150,44 @@ Limitations:
 - no support for `threads` and `atomics`
 - no support for `dynamic libraries`
 
+### WAT WASI app
+
+Create `hello.wat`:
+```
+(module
+    ;; wasi_unstable!fd_write(file_descriptor, *iovs, iovs_len, nwritten) -> status_code
+    (import "wasi_unstable" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))
+
+    (memory 1)
+    (export "memory" (memory 0))
+
+    ;; Put a message to linear memory at offset 32
+    (data (i32.const 32) "Hello, world\n")
+
+    (func $main (export "_start")
+        ;; Create a new io vector
+        (i32.store (i32.const 0) (i32.const 32))  ;; iov.iov_base - pointer to the start of the message
+        (i32.store (i32.const 4) (i32.const 13))  ;; iov.iov_len  - length of the message
+
+        (call $fd_write
+            (i32.const 1)  ;; file_descriptor - 1 for stdout
+            (i32.const 0)  ;; *iovs           - pointer to the io vector
+            (i32.const 1)  ;; iovs_len        - count of items in io vector
+            (i32.const 20) ;; nwritten        - where to store the number of bytes written
+        )
+        drop ;; discard the WASI status code
+)
+```
+
+
+Build and run:
+```sh
+$ wat2wasm hello.wat -o hello.wasm
+
+$ wasm3 hello.wasm    
+Hello, world!
+```
+
 ### WAT library
 
 Create `swap.wat`: