A mold is an idempotent function that coerces a noun to be of a specific type or crashes.

The simplest molds to understand are arms of cores created with +$

+$  height  [feet=@ud inches=@ud]

A mold is compiled to a gate that takes in any noun and produces a typed value, or crashes:

(height [5 11])
::  produces [feet=5 inches=11]

(height %wrong)
::  crashes

To coerce using a gate, it's good practice to use the ;; rune, which can parse inline molds.

;;(height [5 11])
::  produces [feet=5 inches=11]

;;([feet=@ud inches=@ud] [5 11])
::  produces [feet=5 inches=11]

|$ is the mold builder rune which takes a list of molds and produces a mold.

|$ implements parametric polymorphism in Hoon, and as such we call gates produced with |$ a wet gate. We discuss what this means in further detail in the upcoming lesson on polymorphism.

Let's look at some examples from hoon.hoon.

++  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 its 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.