From c5d608216a5090723e795beb6aa53af460c60f07 Mon Sep 17 00:00:00 2001
From: Jeremy Wall <jeremy@marzhillstudios.com>
Date: Tue, 28 May 2019 20:28:40 -0500
Subject: [PATCH] FEATURE: Repl commands.

Adds: #help and #del

Closes #46
---
 src/build/mod.rs  |  4 ++++
 src/help/repl.txt | 18 ++++++++++++++++++
 src/io/mod.rs     |  4 ++++
 src/main.rs       | 37 ++++++++++++++++++++++++++++++++++++-
 4 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 src/help/repl.txt

diff --git a/src/build/mod.rs b/src/build/mod.rs
index 92c2da7..54b437e 100644
--- a/src/build/mod.rs
+++ b/src/build/mod.rs
@@ -295,6 +295,10 @@ where
         self.scope.lookup_sym(&key, true)
     }
 
+    pub fn scope_mut(&mut self) -> &mut Scope {
+        &mut self.scope
+    }
+
     /// Puts the builder in validation mode.
     ///
     /// Among other things this means that assertions will be evaluated and their results
diff --git a/src/help/repl.txt b/src/help/repl.txt
new file mode 100644
index 0000000..da4ac5f
--- /dev/null
+++ b/src/help/repl.txt
@@ -0,0 +1,18 @@
+UCG repl help
+
+The UCG repl allows you to enter ucg statements interactively. You can import
+files create named bindings and see the result of UCG expressions.
+
+The repl understands some non-ucg command as well. Any command that starts with
+a '#' character is treated as a ucg repl command and is not parsed as a ucg
+statement. Repl commands are invalid inside a ucg statement and will result
+in a ucg parse error.
+
+Currently supported commands are:
+
+* #help
+  - prints out this help text
+
+* #del <name>
+  - deletes a previously created named binding
+  
\ No newline at end of file
diff --git a/src/io/mod.rs b/src/io/mod.rs
index 7fcbb58..df38f55 100644
--- a/src/io/mod.rs
+++ b/src/io/mod.rs
@@ -28,6 +28,10 @@ impl StatementAccumulator {
         self.acc.len() + 1
     }
 
+    pub fn last_line(&self) -> Option<&String> {
+        self.acc.last()
+    }
+
     /// Tells you if the latest line ends in the statement terminator.
     ///
     /// Returns None if it wasn't a terminated statement and leaves the
diff --git a/src/main.rs b/src/main.rs
index 8d98967..f2fd49a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -513,6 +513,10 @@ fn env_help() {
     );
 }
 
+fn print_repl_help() {
+    println!(include_str!("help/repl.txt"));
+}
+
 fn do_repl<C: Cache>(
     import_paths: &Vec<PathBuf>,
     cache: Rc<RefCell<C>>,
@@ -552,10 +556,41 @@ fn do_repl<C: Cache>(
     // loop
     let mut lines = ucglib::io::StatementAccumulator::new();
     println!("Welcome to the UCG repl. Ctrl-D to exit");
+    println!("Type '#help' for help.");
     println!("");
     loop {
         // print prompt
-        lines.push(editor.readline(&format!("{}> ", lines.next_line()))?);
+        let line = editor.readline(&format!("{}> ", lines.next_line()))?;
+        // TODO check for a repl command.
+        // repl commands are only valid while not accumulating a statement;
+        let trimmed = line.trim();
+        if trimmed.starts_with("#") {
+            // handle the various commands.
+            if trimmed.starts_with("#help") {
+                print_repl_help();
+            } else if trimmed.starts_with("#del") {
+                // remove a named binding from the builder output.
+                let args: Vec<&str> = trimmed.split(" ").skip(1).collect();
+                if args.len() != 1 {
+                    // print usage of the #del command
+                    eprintln!("The '#del' command expects a single argument specifying \nthe binding to delete.");
+                } else {
+                    let key = ucglib::ast::PositionedItem {
+                        pos: ucglib::ast::Position::new(0, 0, 0),
+                        val: args[0].to_string(),
+                    };
+                    if let None = builder.scope_mut().build_output.remove(&key) {
+                        eprintln!("No such binding {}", key.val);
+                    }
+                }
+            } else {
+                eprintln!("Invalid repl command...");
+                eprintln!("");
+                print_repl_help();
+            }
+            continue;
+        }
+        lines.push(line);
         // check to see if that line is a statement
         loop {
             // read a statement