7. Declare your own effect

Teaches: effect decl, perform, handle end-to-end

Effects aren't limited to the built-ins. effect Net { fetch : Int -> Int } declares a new effect with one operation; with Net as net { fetch(n) => … } installs a handler for it, naming a capability (net) that the program uses to perform fetch calls. This is the moat feature: user-defined effects, not just user-defined data.

effect Net { fetch : Int -> Int }
handle
  (net.fetch(1)) + (net.fetch(2))
with Net as net {
  fetch(n) => n * 10
}

Expected output (bang run stdout):

30

Run it yourself:

curl -fsSL https://raw.githubusercontent.com/phibkro/bang/main/tools/install.sh | sh
bang run examples/handle-custom-tracer/main.bang

← 6. Handling an effect · 8. Swap the handler, keep the program →