Add Solution for #459

Author: coke

solutions/rakudo/Configurable-REPL.md

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+# Configurable REPL Prompt
+
+The default REPL for Rakudo is not configurable.
+
+# Proposed Solution
+
+Implement support for two environment variables:
+
+| Variable | Description |
+|--|--|
+| `RAKUDO_REPL_PROMPT` | the main prompt |
+| `RAKUDO_REPL_PROMPT2` | the continuation prompt |
+
+Each which supports the following interpolations:
+
+| Backslash | Meaning |
+|--|--|
+| `\a` | alert / bell character |
+| `\e` | escape character |
+| `\i` | index - the current iteration of the REPL, which can be referred to later as, e.g. `$*1` |
+| '\l' | `$*RAKU.version` |
+| '\L' | `$*RAKU..gist` |
+| `\t` | time - see below for more options |
+| '\v' | `$*RAKU.compiler.version` |
+| '\V' | `$*RAKU.gistt` |
+
+The escape character allows us to generate ANSI colors, e.g.
+
+`RAKUDO_REPL_PROMPT='[\e[0;31m\i\e[0m] >'
+
+Would generate a prompt similar to the default except the index would appear in red.
+
+## \t formatting
+
+The '\t' construct takes an optional set of '{}' containing a subset of `strftime` codes,
+defaulting to %T if a bare `\t` is used.
+
+| Code | Value |
+|--|--|
+| `%T` | `%H:%M:%S` |
+| `%R` | `%H:%M` |
+| `%H` | 24-Hour hours, 2 digits |
+| `%I` | 12-hour hours, 2 digits |
+| `%M` | minutes, 2 digits |
+| `%S` | seconds, 2 digits |
+
+# Future Development
+
+* Support for colors directly - needs a list of ANSI codes/colors in core.
+* More strftime support - perhaps even invoking it directly instead of manually mapping.
+* Support `RAKUDO_REPL_COMMAND` to support arbitrary code invocation
+
+
+