Imports in WebAssembly

The module states its needs; the host supplies them or no deal.

(module
  ;; Imports are the module's stated needs: the host supplies this
  ;; function at instantiation or the module refuses to start.
  (import "env" "log_i32" (func $log (param i32)))

  (global $reports (mut i32) (i32.const 0))

  (func (export "report") (param $reading i32)
    (global.set $reports
      (i32.add (global.get $reports) (i32.const 1)))
    (call $log (local.get $reading))
    (call $log (global.get $reports))))

How it works

1. (import "env" "log_i32" (func $log ...)) is a two-level name.
2. The import binds to $log and calls like any local function.
3. Instantiation fails outright if the host offers nothing matching.

Keywords and builtins used here

• call
• export
• func
• global
• i32
• import
• module
• mut
• param

The run, in numbers

Lines

12

Characters to type

408

Tokens

79

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 73 seconds.

Type this snippet

Step 3 of 3 in Globals & startup, step 22 of 28 in Language basics.

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