Support for typed functions

[dereckmezquita]

Description

Allow for defining functions with typed arguments and return values. This would require a mechanism for extending base R types.

Implementation Ideas

function2 <- interface("function",
    arg1 = "numeric",
    arg2 = "character",
    return = "numeric"
)

# Implement the function
f2 <- implement(function2, function(arg1, arg2) {
    return(arg1 + nchar(arg2))
})