fix io.debug to behave the same on Erlang and JavaScript (#308)

4 files changed, 25 insertions, 23 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6d2ca82..117efb1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@
 - The `string_builder` module loses the `from_float` function. Use `float.to_string` instead.
 - Fixed the `int.power` and `float.power` functions by properly handling error cases.
 - The grapheme iterator used by `string.graphemes` is now locale independent on target JavaScript.
+- Unified `io.debug` to yield Gleam syntax to standard output (stdout) not just on JavaScript but also Erlang.
 
 ## v0.21.0 - 2022-04-24
 
diff --git a/src/gleam/io.gleam b/src/gleam/io.gleam
index 5a485c9..3e4fadb 100644
--- a/src/gleam/io.gleam
+++ b/src/gleam/io.gleam
@@ -1,3 +1,5 @@
+import gleam/string
+
 /// Writes a string to standard output.
 ///
 /// If you want your output to be printed on its own line see `println`.
@@ -48,7 +50,7 @@ if javascript {
     "../gleam_stdlib.mjs" "log"
 }
 
-/// Prints a value to standard output using Erlang syntax.
+/// Prints a value to standard output (stdout) yielding Gleam syntax.
 ///
 /// The value is returned after being printed so it can be used in pipelines.
 ///
@@ -73,24 +75,9 @@ if javascript {
 /// ```
 ///
 pub fn debug(term: anything) -> anything {
-  debug_print(term)
   term
-}
-
-if erlang {
-  fn debug_print(term: anything) -> DoNotLeak {
-    erlang_fwrite("~tp\n", [term])
-  }
-}
+  |> string.inspect
+  |> println
 
-if javascript {
-  external fn debug_print(anything) -> Nil =
-    "../gleam_stdlib.mjs" "debug"
-}
-
-if erlang {
-  external type DoNotLeak
-
-  external fn erlang_fwrite(String, List(a)) -> DoNotLeak =
-    "io" "fwrite"
+  term
 }
diff --git a/src/gleam/string.gleam b/src/gleam/string.gleam
index d06e2dd..a25bdd7 100644
--- a/src/gleam/string.gleam
+++ b/src/gleam/string.gleam
@@ -6,6 +6,7 @@ import gleam/list
 import gleam/option.{None, Option, Some}
 import gleam/order
 import gleam/string_builder
+import gleam/string_builder.{StringBuilder}
 
 if erlang {
   import gleam/result
@@ -824,17 +825,19 @@ pub fn capitalise(s: String) -> String {
   }
 }
 
-pub fn inspect(value: a) -> String {
-  do_inspect(value)
+/// Returns a `String` representation of values in Gleam syntax.
+///
+pub fn inspect(term: anything) -> String {
+  do_inspect(term)
   |> string_builder.to_string
 }
 
 if javascript {
-  external fn do_inspect(value: a) -> string_builder.StringBuilder =
+  external fn do_inspect(term: anything) -> StringBuilder =
     "../gleam.mjs" "inspect"
 }
 
 if erlang {
-  external fn do_inspect(value: a) -> string_builder.StringBuilder =
+  external fn do_inspect(term: anything) -> StringBuilder =
     "gleam_stdlib" "inspect"
 }
diff --git a/test/gleam/io_test.gleam b/test/gleam/io_test.gleam
new file mode 100644
index 0000000..6408127
--- /dev/null
+++ b/test/gleam/io_test.gleam
@@ -0,0 +1,11 @@
+if erlang {
+  import gleam/io
+  import gleam/should
+
+  pub fn debug_test() {
+    "io.debug-test"
+    // prints to stdout, but EUnit will suppress that:
+    |> io.debug()
+    |> should.equal("io.debug-test")
+  }
+}