Add Solution for Configurable REPL

Fixes #459

solutions/rakudo/Configurable-REPL.md

@@ -0,0 +1,79 @@
+# Configurable REPL Prompt
+
+The default REPL for Rakudo is not configurable.
+
+# Proposed Solution
+
+Implement support for two environment variables:
+
+| Variable | Description | Default |
+|--|--|--|
+| `RAKUDO_REPL_PROMPT` | the main prompt | `[\i] > ` |
+| `RAKUDO_REPL_PROMPT2` | the continuation prompt | `* ` |
+
+Each which supports the following interpolations:
+
+| Sequence | Prompt Expansion |
+|--|--|
+| `\a` | alert / bell character |
+| `\c` | color - see below for options |
+| `\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` | `$*RAKU.gist` |
+| `\n` | newline |
+| `\t` | time - see below for options |
+| `\v` | `$*RAKU.compiler.version` |
+| `\V` | `$*RAKU.gist` |
+
+## \c formatting
+
+Provide some common ANSI codes with short names. Defaults to `reset` if a bare `\c` is used,
+and multiple arguments can be separated with a `;`, e.g.
+
+```
+\c{bold;red}
+```
+
+### formatting
+
+> reset, normal, bold, dim, italic, underline, blink, inverse, hiddne, strikethrough
+
+### foreground colors
+
+> black, red, green, yellow, blue, magenta, cyan, white, default
+
+### background colors
+
+> bg:black, bg:red, bg:green, bg:yellow, bg:blue, bg:magenta, bg:cyan, bg:white, bg:default
+
+Not all escape sequences have aliases - you can always use 
+`\e`  to manually generate codes.
+
+## \t formatting
+
+The `\t` construct takes an optional `{}` 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 |
+
+```
+RAKUDO_REPL_PROMPT='\t{%R} > '
+```
+
+Would generate a prompt containing, e.g. `06:18 > `
+
+# Future Development
+
+* More strftime support - perhaps even invoking it directly instead of manually mapping.
+* Support `RAKUDO_REPL_COMMAND` to support arbitrary code invocation
+
+
+