Seven Obscure Languages in Seven Weeks

Revisiting Deadlocks

There used to be two good friends, Alice and Bob. Once, they had a bitter argument over different dialects of Occam. Chuck, who happened to be their mutual friend and a go-between, told Alice that Bob would send her a text message, but only after she texted him first. Then he told Bob that Alice would send him a text message, but only after he texted her first. The two never spoke to each other again.

The following example translates the drama into Occam:

occam/deadlock.occ

PROC deadlock ()
  VAL hello IS 'H':
  CHAN OF BYTE alice.to.bob, bob.to.alice:
  PAR
    BYTE text.msg:
    SEQ -- Alice: text me, then I text you
      bob.to.alice ? text.msg
      alice.to.bob ! hello

    BYTE text.msg:
    SEQ -- ...