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In the C# example in the code I posted, all cases are matched. If you add a new animal subclass, that's not "explicitly" handled, it will route to the default (

In the C# example in the code I posted, all cases are matched. If you add a new animal subclass, that's not "explicitly" handled, it will route to the default (Animal, Animal) method. It's equivalent of _ -> default_function At the end of a pattern matching statement. Essentially it's impossible for not all the cases to be matched, as far as I can see.

alkonaut 10y ago
Yes but making new classes map to default is what a default clause already does (which is basically what a base case in an abstract base class does with normal inheritance). What I want is to make a new type added mean the code won't compile until it is explicitly handled in all pattern matches that do not have a default clause. Basically what I'm saying is that this: bool HandlePayment(PaymentMethod pm) { switch(pm) { case Invoice(addr): SendInvoice(addr); return true; case CreditCard(ccdetails) return PrcessCreditcard(ccdetails); default: // What? } } Would be a lot better if it didn't require the default, and the compiler could detect the error that occurs when someone adds a new case (BitCoin) to the types of payment method. Inheritance and abstract baseclass for the processing means that you'd do this bool HandlePayment(PaymentMethod pm) { return pm.Process(order); // sends an invoice, charges credit card etc } and you simply have abstract class PaymentMethod { abstract bool Process(order); } class CreditCard : PaymentMethod { ... } class Invoice : PaymentMethod { ... } And this is of course the regular way of writing OO, but I find it unnatural and cumbersome in many cases. It's hard to define concise descriptions of what types are actually available, and you have to write a ton of boilerplate to ensure a closed hierarchy and complete matching in all scenarios where you can't enclose the logic inside each type.

alkonaut 10y ago

Yes but making new classes map to default is what a default clause already does (which is basically what a base case in an abstract base class does with normal inheritance). What I want is to make a new type added mean the code won't compile until it is explicitly handled in all pattern matches that do not have a default clause. Basically what I'm saying is that this: bool HandlePayment(PaymentMethod pm) { switch(pm) { case Invoice(addr): SendInvoice(addr); return true; case CreditCard(ccdetails) return PrcessCreditcard(ccdetails); default: // What? } } Would be a lot better if it didn't require the default, and the compiler could detect the error that occurs when someone adds a new case (BitCoin) to the types of payment method. Inheritance and abstract baseclass for the processing means that you'd do this bool HandlePayment(PaymentMethod pm) { return pm.Process(order); // sends an invoice, charges credit card etc } and you simply have abstract class PaymentMethod { abstract bool Process(order); } class CreditCard : PaymentMethod { ... } class Invoice : PaymentMethod { ... } And this is of course the regular way of writing OO, but I find it unnatural and cumbersome in many cases. It's hard to define concise descriptions of what types are actually available, and you have to write a ton of boilerplate to ensure a closed hierarchy and complete matching in all scenarios where you can't enclose the logic inside each type.