Add basic Expr formatter

src/parse/ast.rs

@@ -1,6 +1,7 @@
 use operator::Operator;
 use parse::problems::Problem;
 use region::Loc;
+use std::fmt::{self, Display, Formatter};
 
 pub type Ident = str;
 pub type VariantName = str;
@@ -157,3 +158,14 @@ pub enum Attempting {
     Module,
     Identifier,
 }
+
+impl<'a> Display for Expr<'a> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        use self::Expr::*;
+
+        match self {
+            EmptyStr => write!(f, "\"\""),
+            _ => panic!("TODO"),
+        }
+    }
+}

tests/test_format.rs

@@ -0,0 +1,51 @@
+#[macro_use]
+extern crate pretty_assertions;
+#[macro_use]
+extern crate indoc;
+extern crate bumpalo;
+extern crate combine; // OBSOLETE
+extern crate roc;
+
+mod helpers;
+
+#[cfg(test)]
+mod test_formatter {
+    use bumpalo::Bump;
+    use roc::parse;
+    use roc::parse::ast::{Attempting, Expr};
+    use roc::parse::parser::{Fail, Parser, State};
+
+    fn parse_with<'a>(arena: &'a Bump, input: &'a str) -> Result<Expr<'a>, Fail> {
+        let state = State::new(&input, Attempting::Module);
+        let parser = parse::expr();
+        let answer = parser.parse(&arena, state);
+
+        answer.map(|(expr, _)| expr).map_err(|(fail, _)| fail)
+    }
+
+    fn assert_formats_to(input: &str, expected: &str) {
+        let arena = Bump::new();
+        let input = input.trim_end();
+        let expected = expected.trim_end();
+
+        match parse_with(&arena, input) {
+            Ok(actual) => assert_eq!(format!("{}", actual), expected),
+            Err(error) => panic!("Unexpected parse failure when parsing this for formatting:\n\n{:?}\n\nParse error was:\n\n{:?}\n\n", input, error)
+        }
+    }
+
+    fn assert_formats_same(input: &str) {
+        assert_formats_to(input, input);
+    }
+
+    // STRING LITERALS
+
+    #[test]
+    fn empty_string() {
+        assert_formats_same(indoc!(
+            r#"
+            ""
+            "#
+        ));
+    }
+}