Include value in error message about invalid value from register_method() (#460)

* Include value in error message about invalid value from `register_method()`

Closes #447

Author: t-kalinowski

R/generic-spec.R

@@ -28,7 +28,8 @@ as_S3_generic <- function(x) {
     }
   }
 
-  stop("`generic` is a function, but not an S3 generic function", call. = FALSE)
+  stop("`generic` is a function, but not an S3 generic function: \n",
+       deparse_trunc(x, 100), call. = FALSE)
 }
 
 S3_generic <- function(generic, name) {

R/utils.R

@@ -144,6 +144,13 @@ modify_list <- function (x, new_vals) {
   x
 }
 
+deparse_trunc <- function(x, width, collapse = "\n") {
+  x <- deparse1(x, collapse)
+  if (nchar(x)> width)
+    x <- sprintf("%s....", substr(x, 0, width-4))
+  x
+}
+
 
 # For older versions of R ----------------------------------------------------
 deparse1 <- function(expr, collapse = " ", width.cutoff = 500L, ...) {

tests/testthat/_snaps/generic-spec.md

@@ -4,7 +4,10 @@
       as_generic(function() { })
     Condition
       Error:
-      ! `generic` is a function, but not an S3 generic function
+      ! `generic` is a function, but not an S3 generic function: 
+      function () 
+      {
+      }
 
 ---