That's basically all you need to know to write threads. The best way to get a good handle on them is just to experiment with some strandio functions. For information on running threads from gall agents, see here and for some examples see here.

Now here's a quick recap of the main points covered:

Spider

is the gall agent that manages threads.
Details of interacting with threads via spider can be seen here.

Threads

are like transient gall agents
are used mostly to chain a series of IO operations
can be used by gall agents to spin out IO operations
live in the ted directory
are managed by the gall agent spider
take a vase and produce a strand which produces a vase

Example

/-  spider
=,  strand=strand:spider
^-  thread:spider
|=  arg=vase
=/  m  (strand ,vase)
^-  form:m
(pure:m arg)

Strands

are the building blocks of threads
take this input and produce this output.
must be specialised to produce a particular type like (strand ,@ud).
are conventionally given the face m.
are a core that has three main arms - form, pure and bind:

form

is the mold of the strand suitable for casting
is the type returned by the other arms

pure

simply returns the form of a strand that produces pure's argument without doing any IO

bind

is used to chain strands together like javascript promises
is used in conjunction with micgal (;<)
must be specialised to a type like ;< <type> bind:m ...
takes two arguments. The first is a function that returns the form of a strand that produces <type>. The second is a gate whose sample is <type> and which returns a form.
calls the first and then, if it succeeded, calls the second with the result of the first as its sample.

Strand input

looks like [=bowl in=(unit input)]
bowl has things like our, now, eny and so forth
bowl is populated once when the thread is first called and then every time it receives new input
input contains any incoming pokes, signs and watches.

Strand output

contains [cards=(list card:agent:gall) <response>]
cards are any cards to be sent immediately
<response> is something like [%done value], [%fail err], etc.
%done will contain the result
responses are only used internally to manage the flow of the thread and are not returned to subscribers.

Strandio

is located in /lib/strandio/hoon
contains a collection of ready-made functions for use in threads
eg. sleep, get-bowl, take-watch, poke, fetch-json, etc.

Summary