Add support for the NO_COLOR environment variable.

CHANGELOG.md

@@ -2,6 +2,7 @@
 All notable changes to this project will be documented in this file.
 
 ## Unreleased - ???
+- Support the `NO_COLOR` environment variable to turn off VT100 color codes in repl (and in scripts).
 - Disallow using `(splice x)` in contexts where it doesn't make sense rather than silently coercing to `x`.
 - Change the names of `:user8` and `:user9` sigals to `:interrupt` and `:await`
 - Change the names of `:user8` and `:user9` fiber statuses to `:interrupted` and `:suspended`.

janet.1

@@ -183,6 +183,10 @@ default repl.
 .BR \-n
 Disable ANSI colors in the repl. Has no effect if no repl is run.
 
+.TP
+.BR \-N
+Enable ANSI colors in the repl. Has no effect if no repl is run.
+
 .TP
 .BR \-r
 Open a REPL (Read Eval Print Loop) after executing all sources. By default, if Janet is called with no
@@ -268,5 +272,11 @@ This variable does nothing in the default configuration of Janet, as PRF is disa
 cannot be defined for this variable to have an effect.
 .RE
 
+.B NO_COLOR
+.RS
+Turn off color by default in the repl and in the error handler of scripts. This can be changed at runtime
+via dynamic bindings *err-color* and *pretty-format*, or via the command line parameters -n and -N.
+.RE
+
 .SH AUTHOR
 Written by Calvin Rose <calsrose@gmail.com>

src/boot/boot.janet

@@ -3855,6 +3855,7 @@
 
   (if-let [jp (getenv-alias "JANET_PATH")] (setdyn *syspath* jp))
   (if-let [jprofile (getenv-alias "JANET_PROFILE")] (setdyn *profilepath* jprofile))
+  (set colorize (not (getenv-alias "NO_COLOR")))
 
   (defn- get-lint-level
     [i]
@@ -3883,6 +3884,7 @@
                -c source output : Compile janet source code into an image
                -i : Load the script argument as an image file instead of source code
                -n : Disable ANSI color output in the REPL
+               -N : Enable ANSI color output in the REPL
                -l lib : Use a module before processing more arguments
                -w level : Set the lint warning level - default is "normal"
                -x level : Set the lint error level - default is "none"
@@ -3898,6 +3900,7 @@
      "i" (fn [&] (set expect-image true) 1)
      "k" (fn [&] (set compile-only true) (set exit-on-error false) 1)
      "n" (fn [&] (set colorize false) 1)
+     "N" (fn [&] (set colorize true) 1)
      "m" (fn [i &] (setdyn *syspath* (in args (+ i 1))) 2)
      "c" (fn c-switch [i &]
            (def path (in args (+ i 1)))