Designing and Instantiating Monads

Due Wednesday, Oct 2nd @ 9 am

Let's begin

1. Pull the latest Exemplars Github Repo
2. Optional META.md

Step-by-step instruction

Context and Motivation

TL;DW:

• This is the last tutorial assignment for this course
• Rest of the coding assignments will be applying the tools we have learned to achieve our goals
• The tasks for this assignment will model how you can design your own runtime system and library as a monad to support an evaluator for your language
• EDIT: We will do the quasiquoter together in class

Recall: An evaluator for our Arith language

TL;DW:

• Evaluating arithmetic might result in an error when we divide by zero.
• We can model the side effects of using an algebraic data type
• Unwrapping and wrapping the ADT is tiresome

Example: The Hopefully Monad

TL;DW:

• We want to map pure functions over non-error'd values, so we define a Functor instance for Hopefully
• We want to be able to accumulate errors that occur in unordered computation, so we define an Applicative instance for Hopefully
• We want to stop performing ordered computation when an argument encounters an error, so we define a Monad instance for Hopefully
• Now we can rewrite our evaluator with do-notation

Your turn: The Logger Monad

TL;DW:

• Repeat the same approach, but for a problem we've seen before: Accumulating string logs during computation.
• The reason you are being asked to implement Functor and Applicative even though our tests only cover the usecase for Monad is because forall f . (Functor f) => (Applicative f) => (Monad f). You cannot actually instance Monad for a datatype without instancing Functor and Applicative for it as well. This wasn't always the case; and you can read more here if you are interested in the history that lead to this design decision

META.md

1. Your name.
2. Your utln.
3. How much time you spent on this assignment.
4. The witness "proof" for how you earned each learning objective with a note of what design pattern you recognized in the recognization objectives.

Please Submit

Contents to be submitted

• Your copy of hw08-1-design-monads.
• META.md

Method of submission