Implement std::str::FromStr for Expression

src/parse.rs

@@ -1,6 +1,7 @@
 use crate::token::Token;
 use crate::expression::Expression;
 use crate::binding::{Binding, Power};
+use crate::lex::TokenSource;
 
 #[derive(Debug)]
 pub enum Error {
@@ -8,6 +9,14 @@ pub enum Error {
 	InvalidSyntax,
 }
 
+impl std::str::FromStr for Expression {
+	type Err = Error;
+
+	fn from_str(s: &str) -> Result<Self, Self::Err> {
+		parse(TokenSource::new(std::io::Cursor::new(s)))
+	}
+}
+
 pub fn parse<I>(input: I) -> Result<Expression, Error>
 where I: Iterator<Item = Result<Token, std::io::Error>>
 {
@@ -484,4 +493,20 @@ mod tests {
 		let result = parse(tokens.into_iter().map(|t| Ok(t)));
 		std::assert_matches!(result, Err(Error::InvalidSyntax));
 	}
+
+	#[test]
+	fn expressions_can_be_read_from_string() {
+		let expr = "x - y".parse::<Expression>().unwrap();
+		assert_eq!(
+			expr,
+			Expression::difference(
+				Expression::Symbol('x'),
+				Expression::Symbol('y')));
+	}
+
+	#[test]
+	fn reading_string_with_invalid_syntax_yields_error() {
+		let result = "(x + 1".parse::<Expression>();
+		assert!(matches!(result, Err(Error::InvalidSyntax)));
+	}
 }