Implement display and string support resolves #10

src/primitives.rs

@@ -1,5 +1,7 @@
 use crate::parser::Datum;
 use crate::parser::Datum::*;
+use std::io;
+use std::io::prelude::*;
 use std::mem;
 
 pub fn add(mut args: Vec<Datum>) -> Datum {
@@ -84,3 +86,17 @@ pub fn lt(args: Vec<Datum>) -> Datum {
         Boolean(true)
     }
 }
+
+pub fn display(args: Vec<Datum>) -> Datum {
+    for i in 1..args.len() {
+        match &args[i] {
+            Boolean(b) => print!("{}", b),
+            Number(n) => print!("{}", n),
+            String(s) => print!("{}", s),
+            List(v) => print!("{:?}", v),
+            _ => panic!("DISPLAY -- cannot-print {:?}", args[1]),
+        };
+    }
+    io::stdout().flush().ok().expect("Could not flush stdout");
+    Unspecified
+}