Repl UX improvements

- Use `"> "` as repl prompt
- Add basic help message at startup
- Skips parsing if an empty line is entered
- Adds previous expressions to linenoise history so that you can get back to them using the arrow keys
- Quit repl on ^C and ^D

src/repl.rs

@@ -77,9 +77,18 @@ fn find_info(args: WithVM<RootStr>) -> IO<Result<String, String>> {
 }
 
 fn compile_repl(vm: &Thread) -> Result<(), Box<StdError + Send + Sync>> {
-    fn input(s: &str) -> IO<Option<String>> {
-        IO::Value(::linenoise::input(s))
+    fn input(prompt: &str) -> IO<Option<String>> {
+        let input = ::linenoise::input(prompt);
+
+        if let Some(ref input) = input {
+            if !input.trim().is_empty() {
+                ::linenoise::history_add(input);
+            }
+        }
+
+        IO::Value(input)
     }
+
     try!(vm.define_global("repl_prim",
                           record!(
         type_of_expr => primitive!(1 type_of_expr),

std/repl.glu

@@ -2,12 +2,10 @@ let prelude = import "std/prelude.glu"
 let map = import "std/map.glu"
 let string = import "std/string.glu"
 let { Map } = map
-let { (>>=), return, (>>), join, map = fmap, lift2, forM_ } = prelude.make_Monad prelude.monad_IO
-let { Eq, Option, Result, Monoid } = prelude
-let { (==) } = string.eq
-let (++) = string.monoid.(<>)
-let { singleton, find, insert, monoid, to_list }
-    = map.make string.ord
+let { (>>=), return, (>>), forM_ } = prelude.make_Monad prelude.monad_IO
+let { Eq, Result, Monoid } = prelude
+let { (<>) = (++) } = string.monoid
+let { singleton, find, monoid, to_list } = map.make string.ord
 let { (<>), empty } = monoid
 
 
@@ -23,35 +21,31 @@ let load_file filename: String -> IO String =
             | Ok expr -> io.load_script modulename expr
             | Err msg -> return msg
 
-type Cmd = { info: String, action: String -> IO Bool }
+type Cmd = {
+    info: String,
+    action: String -> IO Bool
+}
 
 let commands: Map String Cmd =
+    let print_result result =
+        match result with
+            | Ok x -> io.print x
+            | Err x -> io.print x
+
     let commands = ref empty
     let cmds =
         singleton "q" { info = "Quit the REPL", action = \_ -> return False }
             <> singleton "t" {
                 info = "Prints the type with an expression",
-                action = \arg -> repl_prim.type_of_expr arg >>= \result ->
-                    match result with
-                    | Ok x -> io.print x
-                    | Err x -> io.print x
-                        >> return True
+                action = \arg -> repl_prim.type_of_expr arg >>= print_result >> return True
             }
             <> singleton "i" {
                 info = "Prints information about the given name",
-                action = \arg -> repl_prim.find_info arg >>= \result ->
-                    match result with
-                    | Ok x -> io.print x
-                    | Err x -> io.print x
-                        >> return True
+                action = \arg -> repl_prim.find_info arg >>= print_result >> return True
             }
             <> singleton "k" {
                 info = "Prints the kind with the given type",
-                action = \arg -> repl_prim.find_kind arg >>= \result ->
-                    match result with
-                    | Ok x -> io.print x
-                    | Err x -> io.print x
-                        >> return True
+                action = \arg -> repl_prim.find_kind arg >>= print_result >> return True
             }
             <> singleton "l" {
                 info = "Loads the file at 'folder/module.ext' and stores it at 'module'",
@@ -71,7 +65,7 @@ let commands: Map String Cmd =
     commands <- cmds
     load commands
 
-let do_command line: String -> IO Bool = 
+let do_command line: String -> IO Bool =
     let cmd = string.slice line 1 2
     let arg = if string.length line >= 3 then string.trim (string.slice line 3 (string.length line)) else ""
     match find cmd commands with
@@ -81,25 +75,33 @@ let do_command line: String -> IO Bool =
 let store line: String -> IO Bool =
     let line = string.trim line
     match string.find line " " with
-        | Some bind_end -> 
+        | Some bind_end ->
             let binding = string.slice line 0 bind_end
             let expr = string.slice line bind_end (string.length line)
             io.load_script binding expr >> return True
         | None -> io.print "Expected binding in definition" >> return True
 
-let loop x: () -> IO () = repl_prim.input ">>" >>= \line_opt ->
-    match line_opt with
-    | None -> loop ()
-    | Some line ->
-        (if string.starts_with line ":" then
+let loop x: () -> IO () =
+    let run_line line =
+        if string.is_empty (string.trim line) then
+            return True
+        else if string.starts_with line ":" then
             do_command line
         else if string.starts_with line "def " then
             store (string.slice line 4 (string.length line))
         else
-            io.catch (io.run_expr line) return >>= io.print >> return True) >>= \continue -> 
-                if continue then
-                    loop ()
-                else
-                    return ()
+            io.catch (io.run_expr line) return
+                >>= io.print
+                >> return True
+
+    repl_prim.input "> " >>= \line_opt ->
+        match line_opt with
+            | None -> return ()
+            | Some line -> run_line line >>= \continue ->
+                if continue then loop () else return ()
+
+let run x: () -> IO () =
+    io.print "gluon (:h for help, :q to quit)"
+        >>= loop
 
-loop
+run