method-register.md
In S7: An Object Oriented System Meant to Become a Successor to S3 and S4

method registration / adds messages when overwriting

Code
  method(foo, class_character) <- (function(x) "c")
  method(foo, class_character) <- (function(x) "c")
Message
  Overwriting method foo(<character>)

method registration / S3 registration requires a S7 class

Code
  method(sum, new_S3_class("foo")) <- (function(x, ...) "foo")
Condition
  Error:
  ! When registering methods for S3 generic sum(), signature must be an S7 class, not an S3 class.

method registration / can register S7 method for S4 generic

Class has not been registered with S4; please call S4_register(S4foo)

method registration / checks argument types

Code
  x <- 10
  method(x, class_character) <- (function(x) ...)
Condition
  Error:
  ! `generic` must be a function, not a <double>
Code
  method(foo, 1) <- (function(x) ...)
Condition
  Error:
  ! Can't convert `signature` to a valid class. Class specification must be an S7 class object, the result of `new_S3_class()`, an S4 class object, or a base class, not a <double>.

as_signature() / forbids list for single dispatch

Code
  as_signature(list(1), foo)
Condition
  Error:
  ! Can't convert `signature` to a valid class. Class specification must be an S7 class object, the result of `new_S3_class()`, an S4 class object, or a base class, not a <list>.

as_signature() / requires a list of the correct length for multiple dispatch

Code
  as_signature(class_character, foo)
Condition
  Error:
  ! `signature` must be a list for multidispatch generics
Code
  as_signature(list(class_character), foo)
Condition
  Error:
  ! `signature` must be length 2

check_method complains if the functions are not compatible

Code
  foo <- new_generic("foo", "x")
  check_method(1, foo)
Condition
  Error:
  ! foo(???) must be a function
Code
  check_method(function(y) { }, foo)
Condition
  Error:
  ! foo() dispatches on `x`, but foo(???) has arguments `y`
Code
  check_method(function(x = "foo") { }, foo)
Condition
  Error:
  ! In foo(???), dispatch arguments (`x`) must not have default values
Code
  check_method(function(x, y, ...) { }, foo)

Code
  foo <- new_generic("foo", "x", function(x) S7_dispatch())
  check_method(function(x, y) { }, foo)
Condition
  Error:
  ! foo() generic lacks `...` so method formals must match generic formals exactly.
  - generic formals: foo(x)
  - method formals:  foo(x, y)

check_method warn if default arguments don't match

Code
  foo <- new_generic("foo", "x", function(x, ..., z = 2, y = 1) S7_dispatch())
  check_method(function(x, ..., y = 1) { }, foo)
Condition
  Warning:
  foo(???) doesn't have argument `z`
Code
  check_method(function(x, ..., y = 1, z = 1) { }, foo)
Condition
  Warning:
  In foo(???), default value of `z` is not the same as the generic
  - Generic: 2
  - Method:  1

S7_method printing

Code
  method(foo, list(class_integer, class_integer))
Output
  <S7_method> method(foo, list(class_integer, class_integer))
  function (x, y, ...) 
  paste0("bar:", x, y)
  <environment: 0x0>