2.3.2 Molds

A mold is a function that coerces a noun to a type or crashes.

Let's take some examples from hoon.hoon. |$ is a new rune as of the writing of this document so you may see this spelled in slightly different way if you don't have the latest version of hoon.hoon. |$ is the mold builder rune which takes a list of molds and produces a mold.

++  pair
  |$  [head tail]
  [p=head q=tail]

Here is a very simple mold builder. It takes two molds and produces a mold that is a pair of those with the faces p and q. An example of using this would be (pair @ud @ud) which would produce a mold for a cell of @ud and @ud.

++  each
  |$  [this that]
  $%  [%| p=that]
      [%& p=this]
  ==

++each is very slightly more complicated than pair. $% is a rune that is a tagged union. The mold produced by this will match either this or that. An example of this would be (each @ud @tas) which will match either an @ud or a @tas.

++  list
  |$  [item]
  $@(~ [i=item t=(list item)])

Here is a mold builder you've used previously. $@ is a rune that will match the first thing if it's sample is an atom and the second if the sample is a cell. You should be familiar at this point that a list is either ~ or a pair of an item and a list of item.

++  lest
  |$  [item]
  [i/item t/(list item)]

lest you may have heard of as a "non empty list." You can see that it lacks the ~ case that list has but it matches the second part of the list definition. Remember that Hoon types are defined by shape. list could also have been defined in terms of lest like this:

++  list
  |$  [item]
  $@(~ lest)

Another possible way to write it could have been

++  list
  |$  [item]
  $@(~ [i=item t=$])

Any of these would be equivalent.