Pie

This is a tutorial on functional programming in Scala with cats, cats-effect and fs2.

Over the course of the tutorial you'll learn about:

• Algebraic data types
• Structural recursion
• Typeclasses
• Semigroups, monoids, functors and monads
• Monad transformers
• Free encodings
• Effects and evaluation

Getting Started

1. Type sbt to enter the sbt console.
2. Run the application with run 12. This should draw a 12 inch pizza with a single olive (you'll need to use your imagination here).

Take a look inside the pie.PizzaShop to see what gets run.

Algebraic Data Types

Make sure you're familiar with Modelling Data with Traits

Basic validation with the Either datatype

Take a look at the tests in ValidateSizeTest. Write a function validateSize that:

• takes a pizza size as an argument
• evaluates to either a pizza or an error

The error must be one of:

• NegativeSize
• PizzaTooBig
• PizzaTooSmall

Discuss: Should this really evaluate to a pizza? Could there be a better type to use.

Error handling with the Either datatype

Write a function correction. As an argument, it should take in an error.

• If the error is a PizzaTooBig error, it evaluates to a 16 inch pizza
• If the error is a PizzaTooSmall error, it evaluates to a 3 inch pizza
• If the error is none of the above, it evaluates to that same error

Validation with the cats Validated datatype

Write a function validateSauce that:

• takes a sauce name as an argument
• evaluates to either Bechamel, Tomato or a StrangeSauce error

Use a Validated to evaluate both vaildateSize and validateSauce.