Add string constants

* Implement string parsing in tokenizer
 * Implement escape sequences `\"` and `\\`
 * Document changes
 * Update change log

Relates to #35

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 ### Added
 
+ * String constants
+
 ### Removed
 
 ### Changed

README.md

@@ -221,12 +221,12 @@ They return the result as the type it was passed into the function.
 ### Values
 
 Operators take values as arguments and produce values as results.
-Values can be boolean, integer or floating point numbers, tuples or the empty type.
-Strings are supported as well, but there are no operations defined for them yet.
+Values can be boolean, integer or floating point numbers, strings, tuples or the empty type.
 Values are denoted as displayed in the following table.
 
 | Value type | Example |
 |------------|---------|
+| `Value::String` | `"abc"`, `""`, `"a\"b\\c"` |
 | `Value::Boolean` | `true`, `false` |
 | `Value::Int` | `3`, `-9`, `0`, `135412` |
 | `Value::Float` | `3.`, `.35`, `1.00`, `0.5`, `123.554` |

src/context/mod.rs

@@ -1,9 +1,9 @@
 use std::collections::HashMap;
 
-use EvalexprError;
-use EvalexprResult;
 use function::Function;
 use value::value_type::ValueType;
+use EvalexprError;
+use EvalexprResult;
 
 use crate::value::Value;
 

src/error/display.rs

@@ -82,6 +82,7 @@ impl fmt::Display for EvalexprError {
                 write!(f, "Error modulating {} % {}", dividend, divisor)
             },
             ContextNotManipulable => write!(f, "Cannot manipulate context"),
+            IllegalEscapeSequence(string) => write!(f, "Illegal escape sequence: {}", string),
             CustomMessage(message) => write!(f, "Error: {}", message),
         }
     }

src/error/mod.rs

@@ -6,7 +6,7 @@
 //! They are meant as shortcuts to not write the same error checking code everywhere.
 
 use token::PartialToken;
-use value::{TupleType, value_type::ValueType};
+use value::{value_type::ValueType, TupleType};
 
 use crate::value::Value;
 
@@ -163,6 +163,9 @@ pub enum EvalexprError {
     /// A modification was attempted on a `Context` that does not allow modifications.
     ContextNotManipulable,
 
+    /// An escape sequence within a string literal is illegal.
+    IllegalEscapeSequence(String),
+
     /// A custom error explained by its message.
     CustomMessage(String),
 }

src/lib.rs

@@ -208,12 +208,12 @@
 //! ### Values
 //!
 //! Operators take values as arguments and produce values as results.
-//! Values can be boolean, integer or floating point numbers, tuples or the empty type.
-//! Strings are supported as well, but there are no operations defined for them yet.
+//! Values can be boolean, integer or floating point numbers, strings, tuples or the empty type.
 //! Values are denoted as displayed in the following table.
 //!
 //! | Value type | Example |
 //! |------------|---------|
+//! | `Value::String` | `"abc"`, `""`, `"a\"b\\c"` |
 //! | `Value::Boolean` | `true`, `false` |
 //! | `Value::Int` | `3`, `-9`, `0`, `135412` |
 //! | `Value::Float` | `3.`, `.35`, `1.00`, `0.5`, `123.554` |
@@ -361,7 +361,7 @@ pub use function::Function;
 pub use interface::*;
 pub use tree::Node;
 pub use value::{
-    EMPTY_VALUE, EmptyType, FloatType, IntType, TupleType, Value, value_type::ValueType,
+    value_type::ValueType, EmptyType, FloatType, IntType, TupleType, Value, EMPTY_VALUE,
 };
 
 mod context;

src/token/display.rs

@@ -38,6 +38,7 @@ impl fmt::Display for Token {
             Float(float) => float.fmt(f),
             Int(int) => int.fmt(f),
             Boolean(boolean) => boolean.fmt(f),
+            String(string) => fmt::Debug::fmt(string, f),
         }
     }
 }

src/token/mod.rs

@@ -38,6 +38,7 @@ pub enum Token {
     Float(FloatType),
     Int(IntType),
     Boolean(bool),
+    String(String),
 }
 
 #[derive(Clone, Debug, PartialEq)]
@@ -63,6 +64,12 @@ fn char_to_partial_token(c: char) -> PartialToken {
         '%' => PartialToken::Token(Token::Percent),
         '^' => PartialToken::Token(Token::Hat),
 
+        '(' => PartialToken::Token(Token::LBrace),
+        ')' => PartialToken::Token(Token::RBrace),
+
+        ',' => PartialToken::Token(Token::Comma),
+        ';' => PartialToken::Token(Token::Semicolon),
+
         '=' => PartialToken::Eq,
         '!' => PartialToken::ExclamationMark,
         '>' => PartialToken::Gt,
@@ -70,12 +77,6 @@ fn char_to_partial_token(c: char) -> PartialToken {
         '&' => PartialToken::Ampersand,
         '|' => PartialToken::VerticalBar,
 
-        '(' => PartialToken::Token(Token::LBrace),
-        ')' => PartialToken::Token(Token::RBrace),
-
-        ',' => PartialToken::Token(Token::Comma),
-        ';' => PartialToken::Token(Token::Semicolon),
-
         c => {
             if c.is_whitespace() {
                 PartialToken::Whitespace
@@ -118,6 +119,7 @@ impl Token {
             Token::Float(_) => true,
             Token::Int(_) => true,
             Token::Boolean(_) => true,
+            Token::String(_) => true,
         }
     }
 
@@ -152,14 +154,52 @@ impl Token {
             Token::Float(_) => true,
             Token::Int(_) => true,
             Token::Boolean(_) => true,
+            Token::String(_) => true,
         }
     }
 }
 
+/// Parses an escape sequence within a string literal.
+fn parse_escape_sequence<Iter: Iterator<Item = char>>(iter: &mut Iter) -> EvalexprResult<char> {
+    match iter.next() {
+        Some('"') => Ok('"'),
+        Some('\\') => Ok('\\'),
+        Some(c) => Err(EvalexprError::IllegalEscapeSequence(format!("\\{}", c))),
+        None => Err(EvalexprError::IllegalEscapeSequence(format!("\\"))),
+    }
+}
+
+/// Parses a string value from the given character iterator.
+///
+/// The first character from the iterator is interpreted as first character of the string.
+/// The string is terminated by a double quote `"`.
+/// Occurrences of `"` within the string can be escaped with `\`.
+/// The backslash needs to be escaped with another backslash `\`.
+fn parse_string_literal<Iter: Iterator<Item = char>>(
+    mut iter: &mut Iter,
+) -> EvalexprResult<PartialToken> {
+    let mut result = String::new();
+
+    while let Some(c) = iter.next() {
+        match c {
+            '"' => break,
+            '\\' => result.push(parse_escape_sequence(&mut iter)?),
+            c => result.push(c),
+        }
+    }
+
+    Ok(PartialToken::Token(Token::String(result)))
+}
+
 /// Converts a string to a vector of partial tokens.
-fn str_to_tokens(string: &str) -> Vec<PartialToken> {
+fn str_to_partial_tokens(string: &str) -> EvalexprResult<Vec<PartialToken>> {
     let mut result = Vec::new();
-    for c in string.chars() {
+    let mut iter = string.chars().peekable();
+
+    while let Some(c) = iter.next() {
+        if c == '"' {
+            result.push(parse_string_literal(&mut iter)?);
+        } else {
             let partial_token = char_to_partial_token(c);
 
             let if_let_successful =
@@ -176,7 +216,8 @@ fn str_to_tokens(string: &str) -> Vec<PartialToken> {
                 result.push(partial_token);
             }
         }
-    result
+    }
+    Ok(result)
 }
 
 /// Resolves all partial tokens by converting them to complex tokens.
@@ -255,5 +296,5 @@ fn partial_tokens_to_tokens(mut tokens: &[PartialToken]) -> EvalexprResult<Vec<T
 }
 
 pub(crate) fn tokenize(string: &str) -> EvalexprResult<Vec<Token>> {
-    partial_tokens_to_tokens(&str_to_tokens(string))
+    partial_tokens_to_tokens(&str_to_partial_tokens(string)?)
 }

src/tree/mod.rs

@@ -1,9 +1,9 @@
+use token::Token;
+use value::{TupleType, EMPTY_VALUE};
 use EmptyContext;
 use EmptyType;
 use FloatType;
 use IntType;
-use token::Token;
-use value::{EMPTY_VALUE, TupleType};
 
 use crate::{
     context::Context,
@@ -397,9 +397,10 @@ pub(crate) fn tokens_to_operator_tree(tokens: Vec<Token>) -> EvalexprResult<Node
                 }
                 result
             },
-            Token::Float(number) => Some(Node::new(Const::new(Value::Float(number)))),
-            Token::Int(number) => Some(Node::new(Const::new(Value::Int(number)))),
+            Token::Float(float) => Some(Node::new(Const::new(Value::Float(float)))),
+            Token::Int(int) => Some(Node::new(Const::new(Value::Int(int)))),
             Token::Boolean(boolean) => Some(Node::new(Const::new(Value::Boolean(boolean)))),
+            Token::String(string) => Some(Node::new(Const::new(Value::String(string)))),
         };
 
         if let Some(node) = node {

tests/integration.rs

@@ -1,6 +1,6 @@
 extern crate evalexpr;
 
-use evalexpr::{*, error::*};
+use evalexpr::{error::*, *};
 
 #[test]
 fn test_unary_examples() {
@@ -539,6 +539,20 @@ fn test_expression_chaining() {
     );
 }
 
+#[test]
+fn test_strings() {
+    let mut context = HashMapContext::new();
+    assert_eq!(eval("\"string\""), Ok(Value::from("string")));
+    assert_eq!(
+        eval_with_context_mut("a = \"a string\"", &mut context),
+        Ok(Value::Empty)
+    );
+    assert_eq!(
+        eval_boolean_with_context("a == \"a string\"", &context),
+        Ok(true)
+    );
+}
+
 #[cfg(feature = "serde")]
 #[test]
 fn test_serde() {