λMaster Haskell Typeclasses: From Functor to Monad
Climb the typeclass ladder one rung at a time - Eq, Show, Functor, Applicative, Monad - until real Haskell stops looking like runes, and you can ship a tiny expression DSL built on your own custom class.
Phase 1Typeclasses as Everyday Interfaces
See typeclasses as interfaces, not scary math.
A typeclass is a promise, not a type
6 minA typeclass is a promise, not a type
Haskell solves overloading without inheritance
6 minTypeclasses give you overloading without a class hierarchy
Every `=>` is a bouncer checking IDs
7 minConstraints on the left of `=>` are capability checks, not types
The classes you'll actually use, ranked
7 minEight typeclasses cover 90% of real Haskell
Phase 2Writing Your Own Instances
Write instances for Eq, Ord, and Show.
`deriving Eq` is free - until it isn't
7 minDerived Eq is structural; sometimes you need it to be semantic
`compare` is one function, not three
6 minImplementing `compare` once gives you `<`, `<=`, `>`, and `>=` for free
`show` isn't for users - it's for you
7 min`Show` is a debugging tool; user output belongs to your own formatter
A typeclass without laws is just a naming scheme
7 minTypeclass laws are how instances earn their reuse
Build a `Point` that earns Eq, Ord, and Show
8 minTen lines of instance code unlocks the entire standard library
Phase 3The Functor-Applicative-Monad Ladder
Follow the Functor-Applicative-Monad ladder through real code.
Your ops engineer leaves logs only in a `Maybe`
8 minWhen you need to transform a value inside a context without unwrapping it, reach for `fmap`
Three form fields, three `Maybe`s, one user
8 minWhen you need to combine multiple wrapped values with a pure function, reach for `<*>`
`lookup` chain that finally makes `>>=` click
8 minWhen later computations depend on earlier values, reach for `>>=`
Spot the ladder step in any real code
8 minFunctor transforms, Applicative combines, Monad sequences with dependency
Phase 4Ship a Tiny Expression DSL
Design and ship a small expression DSL.
Build a `Pretty` typeclass and print expressions with it
20 minA custom typeclass is just a contract for your own domain
Frequently asked questions
- What's the difference between a typeclass and an interface?
- This is covered in the “Master Haskell Typeclasses: From Functor to Monad” learning path. Start with daily 5-minute micro-lessons that build from fundamentals to hands-on application.
- Do I really have to learn Functor before Monad?
- This is covered in the “Master Haskell Typeclasses: From Functor to Monad” learning path. Start with daily 5-minute micro-lessons that build from fundamentals to hands-on application.
- Why does Haskell want Applicative between Functor and Monad?
- This is covered in the “Master Haskell Typeclasses: From Functor to Monad” learning path. Start with daily 5-minute micro-lessons that build from fundamentals to hands-on application.
- Can I define my own typeclass without losing my mind?
- This is covered in the “Master Haskell Typeclasses: From Functor to Monad” learning path. Start with daily 5-minute micro-lessons that build from fundamentals to hands-on application.
- Is Monad actually about side effects, or something else?
- This is covered in the “Master Haskell Typeclasses: From Functor to Monad” learning path. Start with daily 5-minute micro-lessons that build from fundamentals to hands-on application.
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