Print method for memoised functions

DESCRIPTION

@@ -9,5 +9,7 @@ URL: https://github.com/hadley/memoise
 BugReports: https://github.com/hadley/memoise/issues
 Imports:
     digest
-Suggests: testthat
+Suggests:
+    testthat
 License: MIT + file LICENSE
+RoxygenNote: 5.0.0

NAMESPACE

@@ -1,5 +1,6 @@
-# Generated by roxygen2 (4.1.1): do not edit by hand
+# Generated by roxygen2: do not edit by hand
 
+S3method(print,memoised)
 export(forget)
 export(is.memoised)
 export(is.memoized)

NEWS.md

@@ -1,5 +1,7 @@
 # Version 0.2.99.9000
 
+* Memoised functions now have a print method which displays the original
+  function definition, rather than the memoisation code (#15, @jimhester).
 * A memoised function now has the same interface as the original function,
   if the original function is known when `memoise` is called. (Otherwise,
   the old behavior is invoked, with a warning.) (#14, @krlmlr)

R/memoise.r

@@ -141,6 +141,8 @@ memoise_new <- function(f, envir) {
   memo_f_env$digest <- digest
   environment(memo_f) <- memo_f_env
 
+  class(memo_f) <- c("memoised", "function")
+
   memo_f
 }
 
@@ -168,10 +170,16 @@ memoise_old <- function(f) {
       invisible(res$value)
     }
   }
-  attr(memo_f, "memoised") <- TRUE
+  class(memo_f) <- c("memoised", "function")
   memo_f
 }
 
+#' @export
+print.memoised <- function(x, ...) {
+  cat("Memoised Function:\n")
+  print(environment(x)$f)
+}
+
 #' Forget past results.
 #' Resets the cache of a memoised function.
 #'
@@ -211,5 +219,5 @@ forget <- function(f) {
 #' is.memoised(lm) # FALSE
 #' is.memoised(mem_lm) # TRUE
 is.memoised <- is.memoized <- function(f) {
-  is.function(f) && identical(attr(f, "memoised"), TRUE)
+  is.function(f) && inherits(f, "memoised")
 }

tests/testthat/test-memoise.R

@@ -165,3 +165,19 @@ test_that("can memoise primitive", {
   expect_equal(fm(2, 3), 2 + 3)
   expect_equal(fm(1, 2), 1 + 2)
 })
+
+test_that("printing a memoised function prints the original definition", {
+
+  fn <- function(j) { i <<- i + 1; i }
+
+  fnm <- memoise(fn)
+
+  fn_output <- capture.output(fn)
+  str(fn_output)
+  fnm_output <- capture.output(fnm)
+  str(fnm_output)
+
+  expect_equal(fnm_output[1], "Memoised Function:")
+
+  expect_equal(fnm_output[-1], fn_output)
+})