determine compose_trans domains by pushing domain forward/back throug…

…h transforms (#408)

NEWS.md

@@ -27,6 +27,7 @@
 * Add an inverse (area) hyperbolic sine transformation `asinh_trans()`, which
   provides a logarithm-like transformation of a space, but which accommodates
   negative values (#297)
+* Correct the domain calculation for `compose_trans()` (@mjskay, #408).
 * Transformation objects can optionally include the derivatives of the transform
   and the inverse transform (@mjskay, #322).
 

R/trans-compose.R

@@ -16,10 +16,22 @@ compose_trans <- function(...) {
     cli::cli_abort("{.fun compose_trans} must include at least 1 transformer to compose")
   }
 
-  # Resolve domains
-  suppressWarnings(
-    domain <- compose_fwd(trans_list[[1]]$domain, trans_list[-1])
-  )
+  # Resolve domains. First push the domain of the first transformation all the
+  # way forward through the sequence of transformations, intersecting it with
+  # all domains along the way, to get the range. Then push the range back
+  # through the inverses to get the domain.
+  range <- trans_list[[1]]$transform(trans_list[[1]]$domain)
+  for (trans in trans_list[-1]) {
+    lower <- max(min(trans$domain), min(range))
+    upper <- min(max(trans$domain), max(range))
+    if (isTRUE(lower <= upper)) {
+      range <- trans$transform(c(lower, upper))
+    } else {
+      range <- c(NA_real_, NA_real_)
+      break
+    }
+  }
+  domain <- compose_rev(range, trans_list)
   if (any(is.na(domain))) {
     cli::cli_abort("Sequence of transformations yields invalid domain")
   }

tests/testthat/_snaps/trans-compose.md

@@ -6,7 +6,7 @@
       Error in `compose_trans()`:
       ! `compose_trans()` must include at least 1 transformer to compose
     Code
-      compose_trans("reverse", "log10")
+      compose_trans("sqrt", "reverse", "log10")
     Condition
       Error in `compose_trans()`:
       ! Sequence of transformations yields invalid domain

tests/testthat/test-trans-compose.R

@@ -24,6 +24,15 @@ test_that("uses breaks from first transformer", {
 test_that("produces informative errors", {
   expect_snapshot(error = TRUE, {
     compose_trans()
-    compose_trans("reverse", "log10")
+    compose_trans("sqrt", "reverse", "log10")
   })
 })
+
+test_that("produces correct domains", {
+  expect_equal(compose_trans("sqrt", "reverse")$domain, c(0, Inf))
+  expect_equal(compose_trans("sqrt", "log")$domain, c(0, Inf))
+  expect_equal(compose_trans("log", "log")$domain, c(1, Inf))
+  expect_equal(compose_trans("reverse", "log")$domain, c(-Inf, 0))
+  expect_equal(compose_trans("reverse", "logit", "log")$domain, c(-1, -0.5))
+  expect_error(compose_trans("sqrt", "reverse", "log")$domain, "invalid domain")
+})