This article seems almost entirely concerned with code organization, but in my case I basically never do functional programming and am only concerned about data modelling and data structures. In an adjacent, comment I do acknowledge one point (probably not all software really needs sum types) but I still feel as though languages without sum types are missing an important data and state modelling tool. Trying to take something like a state machine and jam it into interfaces or polymorphism kind of sucks; you lose exhaustiveness checking obviously, but also it enforces a lot of constraints about control flow and data. In Go parlance, if each of my states has a method with its receiver set to the state type, then I can only access that data, so for example I'd need to pass something in or out to handle state transitions. Not ideal IMO.
I feel this pain basically any time I hand-write lexical scanners/parsers in Go or even C++.
> I still feel as though languages without sum types are missing an important data and state modelling tool.
That's because they are. jerf subtly shifts the point to one they can criticise, but it does not change the basic fact that sum types are a critical tool which is just... missing.
There's probably no tool which can't be misused, even the humble boolean, that's not an issue with the tool, and pointing that out is at best irrelevant. It does not change the fact of the matter: you're missing a critical axis of composition. It's like saying screwdrivers are useful but you can misuse them to hammer nails as if that justified trying to screw with a hammer.
Where I'd disagree is that they are a tool, not a critical tool, and they aren't missing from Go, they are simply not the preferred choice. You can cover 75% of the use cases with this approach. It is not 100% of the use cases. But there isn't a language where you can cover 100% of the use cases with 100% effectiveness, which is why we don't have and never will have The One True Language.
Moreover, Go seems to attract this sort of criticism as if Go is Uniquely Broken and it's nirvana in all the other languages... but I've used enough of them to know better. Sum types are great, until you hit the branch of the expression problem where you really need the other side, and if you're in a language that favors them, you're going to get the same 75% experience, just mirror imaged.
> Sum types are great, until you hit the branch of the expression problem where you really need the other side, and if you're in a language that favors them, you're going to get the same 75% experience, just mirror imaged.
Which other side? Product types (aka structs)? I don't think there are any languages with Sum types that don't also include Product types.
"In Go parlance, if each of my states has a method with its receiver set to the state type, then I can only access that data, so for example I'd need to pass something in or out to handle state transitions."
I am not clear what you mean by that, exactly, but generally I model a state machine as a type that composes a state:
type State interface {
isState()
}
// a whole bunch of state types here
type StateMachine struct {
State // exported or not exported, depending on local needs
// additional data
}
func (sm *StateMachine) Event1(args...) error {
// can call current state and change it here
}
You can add an "Execute" method on to the State and have it return an entire state if that's how you want to do it. The state machine can pass in any cross-state data. There's a number of options. You aren't obligated to do exactly, only, and precisely what you'd do in another language, and program X in Y. For some reason, that's common wisdom in the programming world... except for functional programming. I say "don't program X in Y" is simply true and there is no carveout for functional programming in non-FP languages any more than there is the other way around for OO in FP languages.
I feel this pain basically any time I hand-write lexical scanners/parsers in Go or even C++.