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That's a special case for IO, isn't it? I guess I've never actually tried this, but I don't think it's generally the case for all monads. IOW, Haskell does hav

That's a special case for IO, isn't it? I guess I've never actually tried this, but I don't think it's generally the case for all monads. IOW, Haskell does have coloring, and its coloring does manifest through the IO monad, but that does not mean that all monads have color. Sort of like how you can't do, "Socrates is a man, Socrates is dead, therefore all men are dead."

valenterry 6y ago
I think I see what you mean. However, it is the other way around: some monads are special cases in the way that they have extra methods that allow you to "escape" their context. Like lists: you can fold over them and get out a different result type. But in general you can't do that with monads, in particular not if you just have "any" monad at hand but you don't know which one it is. And IO is one example, parsers would be another one.
- aranchelk 6y ago
  I agree with what you’re saying, but I think calling them “special cases” takes away from the elegance of the system. Haskell users frequently say things like “the IO monad” or “the list monad”, but probably the more accurate way would be to say the “the monad instance of IO”. Monad is a typeclass (analogous to an interface in OOP or traits in Rust). IO is a type. To make some type a member of monad (or any other typeclass), you just need to write the required functions in an instance declaration. You can make a type a member of as many typeclasses as you want. List is a member of Foldable, Monad, etc. Calling that a special isn’t quite right as it is very common for a type to be a member of many typeclasses. You’re entirely correct in saying the Monad typeclass offers no way to extract values. But there actually is an extract function called unsafePerformIO with type “IO a -> a”. The name says it all: use it and you’ll lose all of the safety normally afforded by keeping data derived from effects inside the IO type.
  - valenterry 6y ago
    You are totally right, I was a bit sloppy with my language there. Thank you for the detailed explanation.

valenterry 6y ago

I think I see what you mean. However, it is the other way around: some monads are special cases in the way that they have extra methods that allow you to "escape" their context. Like lists: you can fold over them and get out a different result type. But in general you can't do that with monads, in particular not if you just have "any" monad at hand but you don't know which one it is. And IO is one example, parsers would be another one.

aranchelk 6y ago
I agree with what you’re saying, but I think calling them “special cases” takes away from the elegance of the system. Haskell users frequently say things like “the IO monad” or “the list monad”, but probably the more accurate way would be to say the “the monad instance of IO”. Monad is a typeclass (analogous to an interface in OOP or traits in Rust). IO is a type. To make some type a member of monad (or any other typeclass), you just need to write the required functions in an instance declaration. You can make a type a member of as many typeclasses as you want. List is a member of Foldable, Monad, etc. Calling that a special isn’t quite right as it is very common for a type to be a member of many typeclasses. You’re entirely correct in saying the Monad typeclass offers no way to extract values. But there actually is an extract function called unsafePerformIO with type “IO a -> a”. The name says it all: use it and you’ll lose all of the safety normally afforded by keeping data derived from effects inside the IO type.
- valenterry 6y ago
  You are totally right, I was a bit sloppy with my language there. Thank you for the detailed explanation.