Hi R-devels,
I am a bit puzzled about when/if "setIs-conditional" coercion is performed
in S4 method dispatch.
Setup: Myclass1 setIs()-conditionally is inheriting from Myclass2.
My first guess would have been:
If the corresponding test clause in setIs() is TRUE and there is
no Myclass1-method foo() but a Myclass2-method foo(), a call to
foo() with a corresponding Myclass1 instance as argument
automatically coerce this instance to Myclass2 using the coerce
function in setIs(). It does not; (consider the example below).
I am pretty sure that there are good reasons for this behaviour, but could you
give me arguments why this automatic coercion should not be a good idea?
###############################################################################
###### Begin of example
###############################################################################
setClass("Myclass1",representation(a="numeric"))
setClass("Myclass2",representation(b="numeric"))
### conditional inheritance by setIs
setIs("Myclass1", "Myclass2", test = function(obj) [EMAIL PROTECTED]>0,
coerce = function(obj) {cat("setIs coercion used\n")## does not seem to
be used for
new("Myclass2", [EMAIL PROTECTED]) }, ##
automatic coercion
replace = function(obj, value) {cat("setIs replacement used\n") ## does
not seem to be used...
new("Myclass2", [EMAIL PROTECTED])
})
### accessor to slot b
if(!isGeneric("b")) setGeneric("b", function(object) standardGeneric("b"))
setMethod("b","Myclass2", function(object) [EMAIL PROTECTED])
### a method for Myclass2
if(!isGeneric("foo")) setGeneric("foo", function(object) standardGeneric("foo"))
setMethod("foo","Myclass2",function(object)cat("Myclass2-fct:",b(object),"\n"))
###############################################################################
selectMethod("foo","Myclass2") ### does find foo-method for class Myclass2 as
expected
selectMethod("foo","Myclass1") ### does find foo-method for class Myclass2 as
expected
## an instance of Myclass1
MyC1 <- new("Myclass1",a=2)
MyC1.0 <- new("Myclass1",a=-2)
### implicit coercion using setIs coerce function does not work as I would have
expected:
foo(MyC1) ### does not coerce MyC1 to Myclass2 although possible
foo(MyC1.0) ### as expected: does not coerce as not possible
### explicit coercion:
foo(as(MyC1,"Myclass2")) ### works as expected: uses foo-method for class
Myclass2
foo(as(MyC1.0,"Myclass2")) ### works as expected: error as not coercable
### way out?
setMethod("b","Myclass1", function(object)
if ([EMAIL PROTECTED]>0)
[EMAIL PROTECTED]
else
stop("not coercable to class Myclass2")
)
####
foo(MyC1) ### works as expected: uses foo-method for class Myclass2
foo(MyC1.0) ### works as expected: error as not coercable
###############################################################################
###### End of example
###############################################################################
Any suggestion appreciated
thanks already
Peter
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