Add elm language support

Using Haskell's language file as a starting point this add support for
Elm.

rc/extra/elm.kak

@@ -0,0 +1,71 @@
+# http://elm-lang.org
+# ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
+
+# Detection
+# ‾‾‾‾‾‾‾‾‾
+
+hook global BufCreate .*[.](elm) %{
+    set buffer filetype elm
+}
+
+# Highlighters
+# ‾‾‾‾‾‾‾‾‾‾‾‾
+
+add-highlighter -group / regions -default code elm \
+    string   '"'     (?<!\\)(\\\\)*"      '' \
+    comment  (--) $                       '' \
+    comment \{-   -\}                    \{- \
+
+add-highlighter -group /elm/string  fill string
+add-highlighter -group /elm/comment fill comment
+
+add-highlighter -group /elm/code regex \b(import|exposing|as|module|where)\b 0:meta
+add-highlighter -group /elm/code regex \b(True|False)\b 0:value
+add-highlighter -group /elm/code regex \b(if|then|else|case|of|let|in|type|port|alias)\b 0:keyword
+add-highlighter -group /elm/code regex \b(Array|Bool|Char|Float|Int|String)\b 0:type
+
+# Commands
+# ‾‾‾‾‾‾‾‾
+
+# http://elm-lang.org/docs/style-guide
+
+def -hidden elm-filter-around-selections %{
+    # remove trailing white spaces
+    try %{ exec -draft -itersel <a-x> s \h+$ <ret> d }
+}
+
+def -hidden elm-indent-after "
+ exec -draft \\; k x <a-k> ^\\h*(if)|(case\\h+[\\w']+\\h+of|let|in|\\{\\h+\\w+|\\w+\\h+->|[=(])$ <ret> j <a-gt>
+"
+
+def -hidden elm-indent-on-new-line %{
+    eval -draft -itersel %{
+        # copy -- comments prefix and following white spaces
+        try %{ exec -draft k <a-x> s ^\h*\K--\h* <ret> y gh j P }
+        # preserve previous line indent
+        try %{ exec -draft \; K <a-&> }
+        # align to first clause
+        try %{ exec -draft \; k x X s ^\h*(if|then|else)?\h*(([\w']+\h+)+=)?\h*(case\h+[\w']+\h+of|let)\h+\K.* <ret> s \`|.\' <ret> & }
+        # filter previous line
+        try %{ exec -draft k : elm-filter-around-selections <ret> }
+        # indent after lines beginning with condition or ending with expression or =(
+        try %{ elm-indent-after }
+    }
+}
+
+# Initialization
+# ‾‾‾‾‾‾‾‾‾‾‾‾‾‾
+
+hook -group elm-highlight global WinSetOption filetype=elm %{ add-highlighter ref elm }
+
+hook global WinSetOption filetype=elm %{
+    hook window InsertEnd  .* -group elm-hooks  elm-filter-around-selections
+    hook window InsertChar \n -group elm-indent elm-indent-on-new-line
+}
+
+hook -group elm-highlight global WinSetOption filetype=(?!elm).* %{ remove-highlighter elm }
+
+hook global WinSetOption filetype=(?!elm).* %{
+    remove-hooks window elm-indent
+    remove-hooks window elm-hooks
+}