Feature Request: Allow to define S7 functions

[sebffischer]

I want to start this issue with saying how much I enjoy programming in S7 and the ability to directly see the type signature of an object by just looking at the code and not having to read the roxygen docs or scan the function code for stopifnot(inherits(...)).

I was wondering, whether S7 could be extended with a S7_function object, which would bring the same convenience to defining functions, i.e. properly annotated type constraints.

If one were to use :=, similar to how it is discussed here, this could make function definitions look rather clean. Note that below, ClassA would not be the default, but the type of the input x.

f := function(x = ClassA, y = ClassA) {
  x + y
}

which could be internally transformed into something like below, with additional metadata stored in the attributes as is the case for S7.

function(x, y) {
  if (!inherits(x, ClassA)) stop("x must be of classA"),
  if (!inherits(y, ClassA)) stop("x must be of classA")
  x + y
}

for default arguments, one could employ the following syntax (as suggested here).

f := function(x = 1L : ClassA, y = 1L : ClassA) {
  x + y
}
f()
#> 2L

[jtlandis]

Just throwing this out there, this could theoretically enable some return type annotations too. The following syntax is totally valid R syntax

f := function(x = 1L : ClassA, y = 1L : ClassA) {
  x + y
} -> ClassA

[sebffischer]

love it! we could enable dynamically type checking the return type during tests

[sebffischer]

actually I don’t think this will work though, because := is no longer the top level call. Also it would be not nice for readability having the return type at the bottom of a large function.

Maybe we could do something like

f := ClassC ~ function(a = ClassA, b = ClassB) {
 …
}