+++
title = "Child Thread"
weight = 2
template = "doc.html"
+++

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

#### `parent.hoon`

```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`

```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.

```hoon
;<  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`

```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`

```hoon
/-  spider
/+  *strandio
=,  strand=strand:spider
=>
|%
++  url  "https://www.whatsthelatestbasehash.com/"
--
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
;<  =cord  bind:m  (fetch-cord url)
=/  hash-as-cord  `@t`(end [3 (sub (met 3 cord) 1)] cord)
=/  hash  `@uv`(slav %uv hash-as-cord)
(pure:m !>(hash))
```

`child.hoon` simply grabs the latest base hash from https://www.whatsthelatestbasehash.com/ and returns it.

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

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

First we grab the bowl

```hoon
=/  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

```hoon
;<  ~             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.

```hoon
;<  ~             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

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

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

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

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

```hoon
?+  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`

```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`

```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:

```hoon
;<  ~             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.