inspect charlists

Author: lpil

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- The `inspect` function in the `string` module will now print lists of ascii
+  characters in a human readable format, to aid with debugging programs that use
+  Erlang character lists.
 - The deprecated `debug` function in the `io` module has been removed.
 - The deprecated `from` function in the `dynamic` module has been removed.
 

src/gleam_stdlib.erl

@@ -27,9 +27,14 @@
         ((X) >= $a) andalso ((X) =< $f) -> (X) - $a + 10
     end).
 
--define(is_lowercase_char(X), (X > 96 andalso X < 123)).
--define(is_underscore_char(X), (X == 95)).
--define(is_digit_char(X), (X > 47 andalso X < 58)).
+-define(is_lowercase_char(X),
+        (X > 96 andalso X < 123)).
+-define(is_underscore_char(X),
+        (X == 95)).
+-define(is_digit_char(X),
+        (X > 47 andalso X < 58)).
+-define(is_ascii_character(X), 
+        (erlang:is_integer(X) andalso X >= 32 andalso X =< 126)).
 
 uppercase(X) -> X - 32.
 
@@ -289,7 +294,8 @@ inspect(Binary) when is_binary(Binary) ->
 inspect(Bits) when is_bitstring(Bits) ->
     inspect_bit_array(Bits);
 inspect(List) when is_list(List) ->
-    case inspect_list(List) of
+    case inspect_list(List, true) of
+        {charlist, _} -> ["charlist.from_string(\"", list_to_binary(List), "\")"];
         {proper, Elements} -> ["[", Elements, "]"];
         {improper, Elements} -> ["//erl([", Elements, "])"]
     end;
@@ -356,14 +362,17 @@ inspect_maybe_gleam_atom(A, B, C) ->
     erlang:display({A, B, C}),
     throw({gleam_error, A, B, C}).
 
-inspect_list([]) ->
+inspect_list([], _) ->
     {proper, []};
-inspect_list([First]) ->
+inspect_list([First], true) when ?is_ascii_character(First) ->
+    {charlist, nil};
+inspect_list([First], _) ->
     {proper, [inspect(First)]};
-inspect_list([First | Rest]) when is_list(Rest) ->
-    {Kind, Inspected} = inspect_list(Rest),
+inspect_list([First | Rest], ValidCharlist) when is_list(Rest) ->
+    StillValidCharlist = ValidCharlist andalso ?is_ascii_character(First),
+    {Kind, Inspected} = inspect_list(Rest, StillValidCharlist),
     {Kind, [inspect(First), <<", ">> | Inspected]};
-inspect_list([First | ImproperTail]) ->
+inspect_list([First | ImproperTail], _) ->
     {improper, [inspect(First), <<" | ">>, inspect(ImproperTail)]}.
 
 inspect_bit_array(Bits) ->

src/gleam_stdlib.mjs

@@ -9,6 +9,7 @@ import {
   toBitArray,
   bitArraySlice,
   NonEmpty,
+  Empty,
   CustomType,
 } from "./gleam.mjs";
 import { Some, None } from "./gleam/option.mjs";
@@ -780,7 +781,37 @@ function inspectCustomType(record) {
 }
 
 export function inspectList(list) {
-  return `[${list.toArray().map(inspect).join(", ")}]`;
+  if (list instanceof Empty) {
+    return "[]";
+  }
+
+  let char_out = 'charlist.from_string("';
+  let list_out = "[";
+
+  let current = list;
+  while (current instanceof NonEmpty) {
+    let element = current.head;
+    current = current.tail;
+
+    if (list_out !== "[") {
+      list_out += ", ";
+    }
+    list_out += inspect(element);
+
+    if (char_out) {
+      if (Number.isInteger(element) && element >= 32 && element <= 126) {
+        char_out += String.fromCharCode(element);
+      } else {
+        char_out = null;
+      }
+    }
+  }
+
+  if (char_out) {
+    return char_out + '")';
+  } else {
+    return list_out + "]";
+  }
 }
 
 export function inspectUtfCodepoint(codepoint) {

test/gleam/string_test.gleam

@@ -1218,6 +1218,20 @@ pub fn inspect_test() {
   |> should.equal("<<255, 2, 0>>")
 }
 
+pub fn inspect_charlist_test() {
+  let list = [
+    70, 97, 105, 108, 101, 100, 32, 116, 111, 32, 108, 111, 97, 100, 32, 78, 73,
+    70, 32, 108, 105, 98, 114, 97, 114, 121, 58, 32, 39, 47, 114, 117, 110, 47,
+    99, 117, 114, 114, 101, 110, 116, 45, 115, 121, 115, 116, 101, 109, 47, 115,
+    119, 47, 115, 104, 97, 114, 101, 47, 110, 105, 120, 45, 108, 100, 47, 108,
+    105, 98, 47, 108, 105, 98, 99, 114, 121, 112, 116, 111, 46, 115, 111, 46, 51,
+  ]
+  string.inspect(list)
+  |> should.equal(
+    "charlist.from_string(\"Failed to load NIF library: '/run/current-system/sw/share/nix-ld/lib/libcrypto.so.3\")",
+  )
+}
+
 @target(javascript)
 pub fn target_inspect_test() {
   // Due to Erlang's internal representation, on Erlang this passes, instead: