Original Gist

My code diverges from the original gist's code by

converting the unfamiliar Matryoshka-library types and names into the more familiar types and names we've been using here
defining algebras only via "on" (i.e. on symbol function) rather than using "onMatch" (i.e. onMatch {value: function, add: function})
the original paper showed how to define an expression that only allowed Value and Multiply operations. xgrommx's original gist did not have this.

The following table will help you understand the upcoming links to code:

Our Code	xgrommx's code
`newtype Expression f`	`newtype Mu f`
`Expression (VariantF t)`	`MuV t`
`In $ inj symbol data`	`injMu symbol data`
`f a -> a`	`Algebra f a`
`fold`	`cata` `Recursive t f` can be better understood as `t (f t) -> f t`

The links:

Read from ADT1.purs to ADT6.purs: From Algebraic Data Types (ADTs) to Extensible Algebraic Data Types (EADTs). This will help us get used to the different names he uses for the same ideas
The original gist