This folder will do 4 things:
- explain what the Free monad is
- explain how it can be used to create a pure AST and interpret that AST into an impure but useful computation
- explain why one should use
Runinstead ofFree - explain the limitations of
Free/Run.
The below summary and the rest of this folder's contents are quickly explained by Nate in his overview of Free and CoFree: Unrolling Free & Cofree (stop at 1:19:23) (Actual YouTube video name is "PS Unscripted - Free from Tree & Halogen VDOM")
The Free approach deals with the "bind forces us to return the same monad type" problem by using only one monad. Rather than building a large function that is composed of smaller functions (i.e. MTL) that runs once the initial arguments are given to it, the Free approach will create an Abstract Sytax Tree (AST) that describes the desired computation in a pure way. This tree is later "interpreted" via a NaturalTransformation into a base monad (i.e. Effect) that runs those computations in an inpure way. In other words, something akin to
type DSL = DomainSpecificLanguage
type AbstractSyntaxTree output = Free DSL output
defineProgram :: AbstractSyntaxTree
defineProgram = -- implementation
runProgram :: AbstractSyntaxTree ~> Effect
runProgram = -- implementation