Design patterns
Singleton | top-level creation in main, pass it in as a Reader
Factory | Just don't export the contstructor, and do export a smart constructor
Factory Method | Add a type parameter to your ADT
Abstract Factory | Use an existential with a typeclass constraint (aka use a type lambda on the RHS of data decl)
Builder | Add a type parameter to your ADT and add a function parameter to your smart constructor that describes your "special construction process"
Prototype | Don't export constructor, do export *nullary* smart constructor
Object Pool | > Non-optimizing compiler can't even do this for you smh
Chain of Responsibility | Function composition, or some more powerful Monad instance if you need it
Command | Make an ADT for the command
Interpreter | Use Free Monads and Cofree interpreters :P
Iterator | literally the Foldable typeclass
Mediator | Write a typeclass to define the interaction, or use two Free monad DSLs if it is very complex
Observer | >State
Observer (multithreaded) | Use MVars, TVars, or Chans (which are just MVar pairs)
Strategy | Write a typeclass
Template Method | Write a typeclass, plus some default implementations
Visitor | I don't know what this is, but it looks like a Functor
Null Object | Use one of: Either, Maybe, MonadFail m => m
Adapter | Write a newtype and a give it a compatible instance
Bridge, Composite | Category, Arrow, or Lens depending on what you need
Decorator | Write a typeclass instance (I think)
Flyweight | > Mutability. Nice meme
Memento | Just write a function (YourObject -> StateRepr)
Proxy | This is possible, but super contrived and useless in Haskell, so just don't use it