R: Manage Implicit Versions of Generic Functions

implicitGeneric {methods} | R Documentation

## Manage Implicit Versions of Generic Functions

### Description

Create or access implicit generic functions, used to enforce consistent generic versions of functions that are not currently generic. Function `implicitGeneric()` returns the implicit generic version, `setGenericImplicit()` turns a generic implicit, `prohibitGeneric()` prevents your function from being made generic, and `registerImplicitGenerics()` saves a set of implicit generic definitions in the cached table of the current session.

### Usage

    implicitGeneric(name, where, generic)
    setGenericImplicit(name, where, restore = TRUE)
    prohibitGeneric(name, where)
    registerImplicitGenerics(what, where)

### Arguments

`name` | Character string name of the function.
`where` | Package or environment in which to register the implicit generics. When using the functions from the top level of your own package source, this argument can usually be omitted (and should be).
`generic` | Optionally, the generic function definition to be cached, but usually omitted. See Details section.
`restore` | Should the non-generic version of the function be restored after the current.
`what` | For `registerImplicitGenerics()`, Optional table of the implicit generics to register, but nearly always omitted. See Details section.

### Details

Multiple packages may define methods for the same function, using the version of a function stored in one package. All these methods should be marshaled and dispatched consistently when a user calls the function. For consistency, the generic version of the function must have a unique definition (the same arguments allowed in methods signatures, the same values for optional slots such as the value class, and the same standard or non-standard definition of the function itself).

If the original function is already an S4 generic, there is no problem. The implicit generic mechanism enforces consistency when the version in the package owning the function is not generic. If a call to `setGeneric()` attempts to turn a function in another package into a generic, the mechanism compares the proposed new generic function to the implicit generic version of that function. If the two agree, all is well. If not, and if the function belongs to another package, then the new generic will not be associated with that package. Instead, a warning is issued and a separate generic function is created, with its package slot set to the current package, not the one that owns the non-generic version of the function. The effect is that the new package can still define methods for this function, but it will not share the methods in other packages, since it is forcing a different definition of the generic function.

The right way to proceed in nearly all cases is to call `setGeneric("foo")`, giving only the name of the function; this will automatically use the implicit generic version. If you don't like that version, the best solution is to convince the owner of the other package to agree with you and to insert code to define the non-default properties of the function (even if the owner does not want `foo()` to be a generic by default).

For any function, the implicit generic form is a standard generic in which all formal arguments, except for `...`, are allowed in the signature of methods. If that is the suitable generic for a function, no action is needed. If not, the best mechanism is to set up the generic in the code of the package owning the function, and to then call `setGenericImplicit()` to record the implicit generic and restore the non-generic version. See the example.

Note that the package can define methods for the implicit generic as well; when the implicit generic is made a real generic, those methods will be included.

Other than predefining methods, the usual reason for having a non-default implicit generic is to provide a non-default signature, and the usual reason for that is to allow lazy evaluation of some arguments. See the example. All arguments in the signature of a generic function must be evaluated at the time the function needs to select a method. (But those arguments can be missing, with or without a default expression being defined; you can always examine `missing(x)` even for arguments in the signature.)

If you want to completely prohibit anyone from turning your function into a generic, call `prohibitGeneric()`.

### Value

Function `implicitGeneric()` returns the implicit generic definition (and caches that definition the first time if it has to construct it).

The other functions exist for their side effect and return nothing useful.

### See Also

`setGeneric`

### Examples

    ### How we would make the function \link{with}() into a generic:

    ## Since the second argument, 'expr' is used literally, we want
    ## with() to only have "data" in the signature.

    ## Note that 'methods'-internal code now has already extended  with()
    ## to do the equivalent of the following
    ## Not run:
    setGeneric("with", signature = "data")
    ## Now we could predefine methods for "with" if we wanted to.

    ## When ready, we store the generic as implicit, and restore the original
    setGenericImplicit("with")

    ## (This example would only work if we "owned" function with(),
    ##  but it is in base.)
    ## End(Not run)

    implicitGeneric("with")

[Package methods version 3.1.2 Index]