It is true that duck and structural typing have only a subtle difference. But it is also true that duck typing has to be at run time essentially by definition.
Structural typing looks for structural isomorphsims - it is total. Duck typing looks at run time compatibility, so long as the runtime accessed portion is the correct signature it passes.
So structural typing is about isomorphisms and duck typing is about checking the correct boxes before entry. Both have the attribute of looking at structure but you cannot expect them to cluster types in the same manner.
In ML* terms I like to think of structural typing making hard clusters and Duck typing allowing for soft clusters.
Sure if you define the difference away anything can be the same.
But you cannot expect them to always behave the same way. That is, { f_d(t) | t ∈ T} <> {f_s(t) : t ∈ T} where f : T -> T set takes a type and returns the set of types in T equal to it and f_s , f_d check for equivalence via structural and ducks respectively. You can expect f_s to also define a partition over T but not necessarily the same for f_d. f_d is vaguely defined there but if you pretend it acts like it simulates how a runtime check would occur, |f_d| will tend to be >= than |f_s|
An example of how structural typing is not just compile time duck typing is because such a thing exists (in F#) and is not the same as structural typing.
I'm beating a dead horse here, but is there a difference between "compile-time duck-typing" and "structural typing" besides just type inference? It seems to me that so long as the duck-typing language really does unify types, they are the same thing, except that with structural typing you have to write down the type somewhere and it can be a more restrictive type than the actual runtime calls necessitate.
yes, with structural typing, you have to be able to do everything a duck can do, not just the duckish things you're interested in locally. In duck typing, if all you're interested in is walk_like_a_duck, anything with walk_like_a_duck is acceptable, even something that doesn't talk_like_a_duck, because in reality, you never defined the "duck" type to begin with; the actual "duck" type doesn't even exist. In structural typing, "duck" would be a concrete type. "duck typing" is a pretty shitty name for it, to be honest, because of that saying "if it walks like a duck and talks like a duck, it's a duck", but the reality is it's more like "if I need something that walks_like_a_duck and this walks_like_a_duck then it's good enough for me".
let's put it in another perspective: if you have a function that takes disparate types, then the thing that's passed in that function may be of arbitrary type. It's unknown at compile time how much memory will be allocated for the instantiation of that function in order to accept the parameter, because the type that's being passed it is unknown. In structural typing, like in Go, you know at compile time the type of the thing being passed in, and therefore, the amount of resources that will be required upon calling.
Or in another sense, an array of an interface value in Go has a well-defined memory layout, whereas an array of disparate types in a dynamic language does not have a well-defined memory layout at compile time.
internally, an interface value in Go is a two-tuple; it's an actual additional structure; it boxes the structure we're interested in. With duck typing, there is no box.
so yes, it's actually very different. The difference is more meaningful than people generally let on.
Structural typing looks for structural isomorphsims - it is total. Duck typing looks at run time compatibility, so long as the runtime accessed portion is the correct signature it passes.
So structural typing is about isomorphisms and duck typing is about checking the correct boxes before entry. Both have the attribute of looking at structure but you cannot expect them to cluster types in the same manner.
In ML* terms I like to think of structural typing making hard clusters and Duck typing allowing for soft clusters.
* No, not that ML.