Converting from type A to type B creates third type (B/A)

[seandavidross]

Hi. Not sure if this is a bug or a misinterpretation, on my part, of how the package is meant to be used...

How would I get the return value of 'to_sample' (below) to be of type Sample/Whole/numeric -- which is what I expected/hoped for -- rather than Sample/Milliseconds/Whole/numeric (see testthat results pasted below)?

Thanks.

Code

Whole(n) %::% numeric : numeric
Whole(n) %when% {
  n >= 0
} %:=% {
  floor(n)
}


Natural(n) %::% numeric : numeric
Natural(n) %when% {
  is.count(n)
} %:=% {
  n
}


Sample(n) %::% numeric : numeric
Sample(n) %:=%
  Whole(n)


Milliseconds(n) %::% numeric : numeric
Milliseconds(n) %:=%
  Whole(n)


SamplePerSecond(n) %::% numeric : numeric
SamplesPerSecond(n) %:=%
  Natural(n)


to_sample(ms, sample_per_s) %::% Milliseconds : SamplesPerSecond : Sample
to_sample(ms, sample_per_s) %as% {
  per_ms <-
    0.001

  floor(ms * sample_per_s * per_ms) %>% Sample()
}

Failing test result

> library("testthat")
>
> context("to_sample")
> 
> thousand_per_second <-
+   SamplesPerSecond(1000)
> 
> 
> test_that("zero samples can be found in zero milliseconds", {
+   sample_duration <-
+     Milliseconds(0)
+ 
+   to_sample(sample_duration, thousand_per_second) %>%
+     expect_equal(Sample(0))
+ })

Error: Test failed: 'zero samples can be found in zero milliseconds'

• . not equal to Sample(0).
  Classes differ: Sample/Milliseconds/Whole/numeric is not Sample/Whole/numeric

[seandavidross]

answer was to wrap the conversion return value in as.numeric()

[muxspace]

Hi, the main issue is how R manages class information for binary operations. Essentially R is using the class of the first operand:

> thousand_per_second <- SamplesPerSecond(1000)
> sample_duration <- Milliseconds(0)
> sample_duration * thousand_per_second
[1] 0
attr(,"class")
[1] "Milliseconds" "Whole"        "numeric"

> thousand_per_second * sample_duration
[1] 0
attr(,"class")
[1] "SamplesPerSecond" "Natural"          "numeric" 

So when you pass this result to Sample, it's prepending the Sample class to the existing class of the value. And is stripping all of the class information from the value, which is why that works.

FWIW, I generally don't create type hierarchies like this, as I don't think they are necessary. It probably explains why I haven't seen this situation before.