Making illegal states unrepresentable without access to at least HKTs and preferrably also to GADTs and type families is so limited that it becomes almost futile. Or becomes very verbose in a similar way to having no polymorphism/“generics”. #fsharp #Haskell #ddd
Here is a type for a customer and address in pseudo-#fsharp syntax: type Customer f = Customer of Name * f<Adress>. Newly registered customer may or may not enter an address: register : ... -> Customer Option. But...
... placing an order requires the address to be known: placeOrder : ... -> Customer<Identity> -> Order. Where type Identity<a> = a.
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