From 2e67e1220cc855f75c65ec0e3a172833f8fb1ef2 Mon Sep 17 00:00:00 2001
From: Felix Martin <mail@felixm.de>
Date: Sun, 30 May 2021 22:49:31 -0400
Subject: [PATCH] Implement display and string support resolves #10

---
 src/environment.rs |  4 ++++
 src/interpreter.rs |  9 +++++++--
 src/lexer.rs       | 36 ++++++++++++++++++++++++++++++++++++
 src/parser.rs      |  2 ++
 src/primitives.rs  | 16 ++++++++++++++++
 5 files changed, 65 insertions(+), 2 deletions(-)

diff --git a/src/environment.rs b/src/environment.rs
index 94b2bf5..28d2105 100644
--- a/src/environment.rs
+++ b/src/environment.rs
@@ -20,6 +20,10 @@ pub fn get_global_environment<'a>() -> Env<'a> {
             (make_symbol("-"), Datum::Procedure(primitives::sub)),
             (make_symbol("="), Datum::Procedure(primitives::eq)),
             (make_symbol("<"), Datum::Procedure(primitives::lt)),
+            (
+                make_symbol("display"),
+                Datum::Procedure(primitives::display),
+            ),
         ],
         outer: None,
     }
diff --git a/src/interpreter.rs b/src/interpreter.rs
index 4192fe2..798a579 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -83,6 +83,10 @@ fn interpret_define(args: &Vec<Datum>, env: &mut Env) -> Datum {
     }
 }
 
+fn interpret_cond(_vec: &Vec<Datum>, _env: &mut Env) -> Datum {
+    Datum::Unspecified
+}
+
 fn interpret_begin(vec: &Vec<Datum>, env: &mut Env) -> Datum {
     let mut r = Datum::Unspecified;
     for i in 1..vec.len() {
@@ -127,16 +131,17 @@ pub fn interpret(exp: &Datum, env: &mut Env) -> Datum {
     match exp {
         Boolean(_) => exp.clone(),
         Number(_) => exp.clone(),
-        Unspecified => Unspecified,
+        String(_) => exp.clone(),
         Symbol(_) => environment::lookup_variable_value(exp, env),
         List(v) if has_tag(v, "if") => interpret_if(v, env),
         List(v) if has_tag(v, "set!") => interpret_set(v, env),
         List(v) if has_tag(v, "define") => interpret_define(v, env),
-        List(v) if has_tag(v, "cond") => panic!("cond-not-supported"),
+        List(v) if has_tag(v, "cond") => interpret_cond(v, env),
         List(v) if has_tag(v, "let") => panic!("let-not-supported"),
         List(v) if has_tag(v, "begin") => interpret_begin(v, env),
         List(v) if has_tag(v, "quote") => v[1].clone(),
         List(v) => interpret_application(v, env),
+        Unspecified => Unspecified,
         _ => panic!("unknown-expression {:?}", exp),
     }
 }
diff --git a/src/lexer.rs b/src/lexer.rs
index ab19618..4625195 100644
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -3,6 +3,7 @@ pub enum Token {
     Identifier(String),
     Boolean(bool),
     Number(i64),
+    String(String),
     LeftRoundBracket,
     RightRoundBracket,
     Quote,
@@ -33,6 +34,8 @@ fn scan(code: &str, mut ix: usize, mut tokens: Tokens) -> Tokens {
         return scan_boolean(code, ix, tokens);
     } else if c == ';' {
         return scan_comment(code, ix, tokens);
+    } else if c == '"' {
+        return scan_string(code, ix, tokens);
     } else if c.is_ascii_digit() {
         return scan_number(code, ix, tokens);
     } else if c.is_ascii_alphabetic() || SPECIAL_INITIAL.contains(&c) {
@@ -95,6 +98,39 @@ fn scan_comment(code: &str, mut ix: usize, tokens: Tokens) -> Tokens {
     scan(code, ix, tokens)
 }
 
+fn scan_string(code: &str, mut ix: usize, mut tokens: Tokens) -> Tokens {
+    let mut s = String::new();
+    let mut chars = code[ix..].chars();
+    let c = chars.next().unwrap();
+    assert!(c == '"');
+
+    ix += 1;
+    while let Some(c) = chars.next() {
+        ix += 1;
+        if c == '"' {
+            break;
+        } else if c == '\\' {
+            ix += 1;
+            if let Some(next_c) = chars.next() {
+                let escaped_c = match next_c {
+                    'n' => '\n',
+                    '"' => '"',
+                    '\\' => '\\',
+                    _ => panic!("SCAN-STRING -- cannot escape {}", next_c),
+                };
+                s.push(escaped_c);
+            } else {
+                panic!("SCAN-STRING -- found escape at eof")
+            }
+        } else {
+            s.push(c);
+        }
+    }
+    let token = Token::String(s);
+    tokens.push(token);
+    scan(code, ix, tokens)
+}
+
 fn scan_number(code: &str, mut ix: usize, mut tokens: Tokens) -> Tokens {
     let start_ix = ix;
     let mut chars = code[ix..].chars();
diff --git a/src/parser.rs b/src/parser.rs
index 23f748e..b915e6f 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -6,6 +6,7 @@ use std::cmp::Ordering;
 pub enum Datum {
     Boolean(bool),
     Number(i64),
+    String(String),
     Symbol(String),
     List(Vec<Datum>),
     Procedure(fn(Vec<Datum>) -> Datum),
@@ -52,6 +53,7 @@ fn parse_datum(tokens: &Tokens, ix: usize) -> (Datum, usize) {
         Token::Identifier(s) => (Datum::Symbol(s.to_string()), ix + 1),
         Token::Boolean(b) => (Datum::Boolean(*b), ix + 1),
         Token::Number(n) => (Datum::Number(*n), ix + 1),
+        Token::String(s) => (Datum::String(s.to_string()), ix + 1),
         Token::Quote => parse_quote(tokens, ix + 1),
         Token::LeftRoundBracket => parse_list(tokens, ix + 1),
         _ => panic!("Unexpected token {:?}", tokens[ix]),
diff --git a/src/primitives.rs b/src/primitives.rs
index 0cd7744..90b9e7c 100644
--- a/src/primitives.rs
+++ b/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
+}