Clarify x/i error prefixes in merge() for incompatible factor joins

.ci/.lintr.R

@@ -40,6 +40,9 @@ linters = c(dt_linters, all_linters(
   # TODO(lintr#2442): Use this once x[ , j, by] is supported.
   commas_linter = NULL,
   commented_code_linter = NULL,
+  # mostly, we just use stopf() & friends, but ignore this for the
+  #   rare cases we need plain stop (e.g. #7048)
+  condition_call_linter = NULL,
   # TODO(linter->3.2.0): Activate this.
   consecutive_assertion_linter = NULL,
   cyclocomp_linter = NULL,

R/bmerge.R

@@ -84,7 +84,17 @@ bmerge = function(i, x, icols, xcols, roll, rollends, nomatch, mult, ops, verbos
           next
         }
       }
-      stopf("Incompatible join types: %s (%s) and %s (%s). Factor columns must join to factor or character columns.", xname, x_merge_type, iname, i_merge_type)
+      condition_message = gettextf("Incompatible join types: %s (%s) and %s (%s). Factor columns must join to factor or character columns.", xname, x_merge_type, iname, i_merge_type)
+      condition = list(
+        message = condition_message,
+        call = sys.call(sys.nframe() - 1L),
+        bmerge_x_arg_col_name      = names(x)[xcol],
+        bmerge_x_arg_type          = x_merge_type,
+        bmerge_i_arg_col_name      = names(i)[icol],
+        bmerge_i_arg_type          = i_merge_type
+      )
+      class(condition) = c("dt_bmerge_incompatible_type_error", "error", "condition")
+      stop(condition)
     }
     # we check factors first to cater for the case when trying to do rolling joins on factors
     if (x_merge_type == i_merge_type) {

R/merge.R

@@ -90,15 +90,24 @@ merge.data.table = function(x, y, by = NULL, by.x = NULL, by.y = NULL, all = FAL
 
   # implement incomparables argument #2587
   if (!is.null(incomparables)) {
-    # %fin% to be replaced when #5232 is implemented/closed
-    "%fin%" = function(x, table) if (is.character(x) && is.character(table)) x %chin% table else x %in% table
-    xind = rowSums(x[, lapply(.SD, function(x) !(x %fin% incomparables)), .SDcols=by.x]) == length(by)
-    yind = rowSums(y[, lapply(.SD, function(x) !(x %fin% incomparables)), .SDcols=by.y]) == length(by)
-    # subset both so later steps still work
+    "%fin%" = function(x_val, table_val) if (is.character(x_val) && is.character(table_val)) x_val %chin% table_val else x_val %in% table_val
+    xind = rowSums(x[, lapply(.SD, function(x_col_val) !(x_col_val %fin% incomparables)), .SDcols=by.x]) == length(by.x)
+    yind = rowSums(y[, lapply(.SD, function(y_col_val) !(y_col_val %fin% incomparables)), .SDcols=by.y]) == length(by.y)
     x = x[xind]
     y = y[yind]
   }
-  dt = y[x, nomatch=if (all.x) NA else NULL, on=by, allow.cartesian=allow.cartesian]   # includes JIS columns (with a i. prefix if conflict with x names)
+
+  dt = tryCatch(
+    y[x, nomatch=if (all.x) NA else NULL, on=by, allow.cartesian=allow.cartesian],
+    dt_bmerge_incompatible_type_error = function(e) {
+      x_part_col_name = paste0("x.", e$bmerge_i_arg_col_name)
+      x_part_type     = e$bmerge_i_arg_type
+      y_part_col_name = paste0("y.", e$bmerge_x_arg_col_name)
+      y_part_type     = e$bmerge_x_arg_type
+
+      stopf("Incompatible join types: %s (%s) and %s (%s). Factor columns must join to factor or character columns.", x_part_col_name, x_part_type, y_part_col_name, y_part_type)
+    }
+  )
 
   if (all.y && nrow(y)) {  # If y does not have any rows, no need to proceed
     # Perhaps not very commonly used, so not a huge deal that the join is redone here.

inst/tests/tests.Rraw

@@ -21157,3 +21157,8 @@ test(2317.6, DT1[DF1, on='a', .(d = x.a + i.d)]$d, 5)
 test(2317.7, DT1[DF2, on='a', e := i.e]$e, 5)
 test(2317.8, DT1[DF2, on='a', e2 := x.a + i.e]$e2, 6)
 test(2317.9, DT1[DF2, on='a', .(e = x.a + i.e)]$e, 6)
+
+# Test for incompatible factor joins rephrased by merge.data.table(#7048)
+DT1=data.table(a=factor('a'))
+DT2=data.table(a=1L)
+test(2318, merge(DT1, DT2, by = "a"), error = "Incompatible join types: x.a (factor) and y.a (integer). Factor columns must join to factor or character columns.")