Child Thread • Reference • developers.urbit.org

Here's a simple example of a thread that starts another thread:

`parent.hoon`

/-  spider
/+  *strandio
=,  strand=strand:spider
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
;<  tid=tid:spider   bind:m  (start-thread %child)
(pure:m !>(~))

`child.hoon`

/-  spider
/+  *strandio
=,  strand=strand:spider
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
%-  (slog leaf+"foo" ~)
(pure:m !>(~))

Save parent.hoon and child.hoon in /ted of the %base desk, |commit %base and run -parent. You should see something like:

foo
> -parent
~

parent.hoon just uses the strandio function start-thread to start child.hoon, and child.hoon just prints foo to the dojo. Since we got foo we can tell the second thread did, in fact, run.

;<  tid=tid:spider   bind:m  (start-thread %child)

See here how we gave start-thread the name of the thread to run. It returns the tid of the thread, which we could then use to poke it or whatever.

start-thread handles creating the tid for the thread so is quite convenient.

Note that threads we start this way will be a child of the thread that started them, and so will be killed when the parent thread ends.

Start thread and get its result

If we want to actually get the result of the thread we started, it's slightly more complicated. We note that this is mostly the same as await-thread:strandio.

`parent.hoon`

/-  spider
/+  *strandio
=,  strand=strand:spider
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
;<  =bowl:spider  bind:m  get-bowl
=/  tid  `@ta`(cat 3 'strand_' (scot %uv (sham %child eny.bowl)))
;<  ~             bind:m  (watch-our /awaiting/[tid] %spider /thread-result/[tid])
;<  ~             bind:m  %-  poke-our
                          :*  %spider
                              %spider-start
                              !>([`tid.bowl `tid byk.bowl(r da+now.bowl) %child !>(~)])
                          ==
;<  =cage         bind:m  (take-fact /awaiting/[tid])
;<  ~             bind:m  (take-kick /awaiting/[tid])
?+  p.cage  ~|([%strange-thread-result p.cage %child tid] !!)
  %thread-done  (pure:m q.cage)
  %thread-fail  (strand-fail !<([term tang] q.cage))
==

`child.hoon`

/-  spider
/+  *strandio
=,  strand=strand:spider
=>
|%
++  url  "https://sampledataapi.com/API/getcategory"
--
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
;<  =cord  bind:m  (fetch-cord url)
(pure:m !>(cord))

child.hoon simply grabs the JSON response from https://sampledataapi.com/ and returns it as a cord. (There is also a ++fetch-json:strandio gate that could be employed here.)

parent.hoon is a bit more complicated so we'll look at it line-by-line

;<  =bowl:spider  bind:m  get-bowl

First we grab the bowl

=/  tid  `@ta`(cat 3 'strand_' (scot %uv (sham %child eny.bowl)))

Then we generate a tid (thread ID) for the thread we're gonna start

;<  ~             bind:m  (watch-our /awaiting/[tid] %spider /thread-result/[tid])

We pre-emptively subscribe for the result. Spider sends the result at /thread-result/<tid> so that's where we subscribe.

;<  ~             bind:m  %-  poke-our
                          :*  %spider
                              %spider-start
                              !>([`tid.bowl `tid byk.bowl(r da+now.bowl) %child !>(~)])
                          ==

Spider takes a poke with a mark %spider-start and a vase containing [parent=(unit tid) use=(unit tid) =beak file=term =vase] to start a thread, where:

parent is an optional parent thread. In this case we say the parent is our tid. Specifying a parent means the child will be killed if the parent ends.
use is the thread ID for the thread we're creating
beak is a [p=ship q=desk r=case] triple which specifies the desk and revision containing the thread we want to run. In this case we just use byk.bowl, but with the date of revision q changed to now.bowl.
file is the filename of the thread we want to start
vase is the vase it will be given as an argument when it's started

;<  =cage         bind:m  (take-fact /awaiting/[tid])

We wait for a fact which will be the result of the thread.

;<  ~             bind:m  (take-kick /awaiting/[tid])

Spider will kick us from the subscription when it ends the thread so we also take that kick.

?+  p.cage  ~|([%strange-thread-result p.cage %child tid] !!)
  %thread-done  (pure:m q.cage)
  %thread-fail  (strand-fail !<([term tang] q.cage))
==

Finally we test whether the thread produced a %thread-done or a %thread-fail. These are the two possible marks produced by spider when it returns the results of a thread. A %thread-done will contain a vase with the result, and a %thread-fail will contain an error message and traceback, so we see which it is and then either produce the result with pure or trigger a %thread-fail with the error we got from the child.

Stop a thread

`parent.hoon`

/-  spider
/+  *strandio
=,  strand=strand:spider
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
;<  =bowl:spider  bind:m  get-bowl
=/  tid  `@ta`(cat 3 'strand_' (scot %uv (sham %child eny.bowl)))
%-  (slog leaf+"Starting child thread..." ~)
;<  ~             bind:m  %-  poke-our
                          :*  %spider
                              %spider-start
                              !>([`tid.bowl byk.bowl(r da+now.bowl) `tid %child !>(~)])
                          ==
;<  ~             bind:m  (sleep ~s5)
%-  (slog leaf+"Stopping child thread..." ~)
;<  ~             bind:m  %-  poke-our
                          :*  %spider
                              %spider-stop
                              !>([tid %.y])
                          ==
;<  ~             bind:m  (sleep ~s2)
(pure:m !>("Done"))

`child.hoon`

/-  spider
/+  *strandio
=,  strand=strand:spider
=>
|%
++  looper
  =/  m  (strand ,~)
  ^-  form:m
  %-  (main-loop ,~)
  :~  |=  ~
      ^-  form:m
      ;<  ~  bind:m  (sleep `@dr`(div ~s1 2))
      %-  (slog leaf+"child thread" ~)
      (pure:m ~)
  ==
--
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
;<  ~  bind:m  looper
(pure:m !>(~))

child.hoon just prints to the dojo in a loop.

parent.hoon starts child.hoon, and then pokes spider like:

;<  ~             bind:m  %-  poke-our
                          :*  %spider
                              %spider-stop
                              !>([tid %.y])
                          ==

%spider-stop is the mark that tells spider to kill a thread.
tid is the tid of the thread to kill
%.y tells spider to suppress traceback in the result of the killed thread. If you give it %.n it will include the traceback.