Use justfile instead of mf on invalid examples in readme (#2447)

GRAMMAR.md

@@ -90,7 +90,13 @@ import        : 'import' '?'? string? eol
 
 module        : 'mod' '?'? NAME string? eol
 
-expression    : 'if' condition '{' expression '}' 'else' '{' expression '}'
+expression    : disjunct || expression
+              | disjunct
+
+disjunct      : conjunct && disjunct
+              | conjunct
+
+conjunct      : 'if' condition '{' expression '}' 'else' '{' expression '}'
               | 'assert' '(' condition ',' expression ')'
               | '/' expression
               | value '/' expression

README.md

@@ -1290,9 +1290,11 @@ Available recipes:
     test
 ```
 
-### Variables and Substitution
+### Expressions and Substitutions
 
-Variables, strings, concatenation, path joining, substitution using `{{…}}`, and function calls are supported:
+Various operators and function calls are supported in expressions, which may be
+used in assignments, default recipe arguments, and inside recipe body `{{…}}`
+substitutions.
 
 ```just
 tmpdir  := `mktemp -d`
@@ -1310,6 +1312,39 @@ publish:
   rm -rf {{tarball}} {{tardir}}
 ```
 
+#### Concatenation
+
+The `+` operator returns the left-hand argument concatenated with the
+right-hand argument:
+
+```just
+foobar := 'foo' + 'bar'
+```
+
+#### Logical Operators
+
+The logical operators `&&` and `||` can be used to coalesce string
+values<sup>master</sup>, similar to Python's `and` and `or`. These operators
+consider the empty string `''` to be false, and all other strings to be true.
+
+These operators are currently unstable.
+
+The `&&` operator returns the empty string if the left-hand argument is the
+empty string, otherwise it returns the right-hand argument:
+
+```justfile
+foo := '' && 'goodbye'      # ''
+bar := 'hello' && 'goodbye' # 'goodbye'
+```
+
+The `||` operator returns the left-hand argument if it is non-empty, otherwise
+it returns the right-hand argument:
+
+```justfile
+foo := '' || 'goodbye'      # 'goodbye'
+bar := 'hello' || 'goodbye' # 'hello'
+```
+
 #### Joining Paths
 
 The `/` operator can be used to join two strings with a slash:
@@ -2367,8 +2402,8 @@ Testing server:unit…
 ./test --tests unit server
 ```
 
-Default values may be arbitrary expressions, but concatenations or path joins
-must be parenthesized:
+Default values may be arbitrary expressions, but expressions containing the
+`+`, `&&`, `||`, or `/` operators must be parenthesized:
 
 ```just
 arch := "wasm"
@@ -2740,7 +2775,7 @@ pass a Windows-style path to the interpreter.
 Recipe lines are interpreted by the shell, not `just`, so it's not possible to
 set `just` variables in the middle of a recipe:
 
-```mf
+```justfile
 foo:
   x := "hello" # This doesn't work!
   echo {{x}}
@@ -2872,7 +2907,7 @@ means that multi-line constructs probably won't do what you want.
 
 For example, with the following `justfile`:
 
-```mf
+```justfile
 conditional:
   if true; then
     echo 'True!'
@@ -3279,7 +3314,7 @@ One `justfile` can include the contents of another using `import` statements.
 
 If you have the following `justfile`:
 
-```mf
+```justfile
 import 'foo/bar.just'
 
 a: b
@@ -3319,7 +3354,7 @@ set, variables in parent modules override variables in imports.
 
 Imports may be made optional by putting a `?` after the `import` keyword:
 
-```mf
+```just
 import? 'foo/bar.just'
 ```
 
@@ -3328,19 +3363,19 @@ This allows importing multiple justfiles, for example `foo.just` and
 `bar.just`, which both import a third justfile containing shared recipes, for
 example `baz.just`, without the duplicate import of `baz.just` being an error:
 
-```mf
+```justfile
 # justfile
 import 'foo.just'
 import 'bar.just'
 ```
 
-```mf
+```justfile
 # foo.just
 import 'baz.just'
 foo: baz
 ```
 
-```mf
+```justfile
 # bar.just
 import 'baz.just'
 bar: baz
@@ -3361,7 +3396,7 @@ versions, you'll need to use the `--unstable` flag, `set unstable`, or set the
 
 If you have the following `justfile`:
 
-```mf
+```justfile
 mod bar
 
 a:
@@ -3399,7 +3434,7 @@ the module file may have any capitalization.
 
 Module statements may be of the form:
 
-```mf
+```justfile
 mod foo 'PATH'
 ```
 
@@ -3423,7 +3458,7 @@ recipes.
 
 Modules may be made optional by putting a `?` after the `mod` keyword:
 
-```mf
+```just
 mod? foo
 ```
 
@@ -3433,7 +3468,7 @@ Optional modules with no source file do not conflict, so you can have multiple
 mod statements with the same name, but with different source file paths, as
 long as at most one source file exists:
 
-```mf
+```just
 mod? foo 'bar.just'
 mod? foo 'baz.just'
 ```
@@ -3441,7 +3476,7 @@ mod? foo 'baz.just'
 Modules may be given doc comments which appear in `--list`
 output<sup>1.30.0</sup>:
 
-```mf
+```justfile
 # foo is a great module!
 mod foo
 ```
@@ -3583,9 +3618,9 @@ The following command will create two files, `some` and `argument.txt`:
 $ just foo "some argument.txt"
 ```
 
-The users shell will parse `"some argument.txt"` as a single argument, but when
-`just` replaces `touch {{argument}}` with `touch some argument.txt`, the quotes
-are not preserved, and `touch` will receive two arguments.
+The user's shell will parse `"some argument.txt"` as a single argument, but
+when `just` replaces `touch {{argument}}` with `touch some argument.txt`, the
+quotes are not preserved, and `touch` will receive two arguments.
 
 There are a few ways to avoid this: quoting, positional arguments, and exported
 arguments.
@@ -3910,6 +3945,38 @@ fetch:
 Given the above `justfile`, after running `just fetch`, the recipes in
 `foo.just` will be available.
 
+### Printing Complex Strings
+
+`echo` can be used to print strings, but because it processes escape sequences,
+like `\n`, and different implementations of `echo` recognize different escape
+sequences, using `printf` is often a better choice.
+
+`printf` takes a C-style format string and any number of arguments, which are
+interpolated into the format string.
+
+This can be combined with indented, triple quoted strings to emulate shell
+heredocs.
+
+Substitution complex strings into recipe bodies with `{…}` can also lead to
+trouble as it may be split by the shell into multiple arguments depending on
+the presence of whitespace and quotes. Exporting complex strings as environment
+variables and referring to them with `"$NAME"`, note the double quotes, can
+also help.
+
+Putting all this together, to print a string verbatim to standard output, with
+all its various escape sequences and quotes undisturbed:
+
+```just
+export FOO := '''
+  a complicated string with
+  some dis\tur\bi\ng escape sequences
+  and "quotes" of 'different' kinds
+'''
+
+bar:
+  printf %s "$FOO"
+```
+
 ### Alternatives and Prior Art
 
 There is no shortage of command runners! Some more or less similar alternatives

src/analyzer.rs

@@ -36,12 +36,15 @@ impl<'run, 'src> Analyzer<'run, 'src> {
   ) -> CompileResult<'src, Justfile<'src>> {
     let mut definitions = HashMap::new();
     let mut imports = HashSet::new();
+    let mut unstable_features = BTreeSet::new();
 
     let mut stack = Vec::new();
     let ast = asts.get(root).unwrap();
     stack.push(ast);
 
     while let Some(ast) = stack.pop() {
+      unstable_features.extend(&ast.unstable_features);
+
       for item in &ast.items {
         match item {
           Item::Alias(alias) => {
@@ -166,8 +169,6 @@ impl<'run, 'src> Analyzer<'run, 'src> {
       aliases.insert(Self::resolve_alias(&recipes, alias)?);
     }
 
-    let mut unstable_features = BTreeSet::new();
-
     for recipe in recipes.values() {
       for attribute in &recipe.attributes {
         if let Attribute::Script(_) = attribute {

src/assignment_resolver.rs

@@ -31,7 +31,29 @@ impl<'src: 'run, 'run> AssignmentResolver<'src, 'run> {
     self.stack.push(name);
 
     if let Some(assignment) = self.assignments.get(name) {
-      self.resolve_expression(&assignment.value)?;
+      for variable in assignment.value.variables() {
+        let name = variable.lexeme();
+
+        if self.evaluated.contains(name) || constants().contains_key(name) {
+          continue;
+        }
+
+        if self.stack.contains(&name) {
+          self.stack.push(name);
+          return Err(
+            self.assignments[name]
+              .name
+              .error(CircularVariableDependency {
+                variable: name,
+                circle: self.stack.clone(),
+              }),
+          );
+        } else if self.assignments.contains_key(name) {
+          self.resolve_assignment(name)?;
+        } else {
+          return Err(variable.error(UndefinedVariable { variable: name }));
+        }
+      }
       self.evaluated.insert(name);
     } else {
       let message = format!("attempted to resolve unknown assignment `{name}`");
@@ -51,112 +73,6 @@ impl<'src: 'run, 'run> AssignmentResolver<'src, 'run> {
 
     Ok(())
   }
-
-  fn resolve_expression(&mut self, expression: &Expression<'src>) -> CompileResult<'src> {
-    match expression {
-      Expression::Assert {
-        condition: Condition {
-          lhs,
-          rhs,
-          operator: _,
-        },
-        error,
-      } => {
-        self.resolve_expression(lhs)?;
-        self.resolve_expression(rhs)?;
-        self.resolve_expression(error)
-      }
-      Expression::Call { thunk } => match thunk {
-        Thunk::Nullary { .. } => Ok(()),
-        Thunk::Unary { arg, .. } => self.resolve_expression(arg),
-        Thunk::UnaryOpt { args: (a, b), .. } => {
-          self.resolve_expression(a)?;
-          if let Some(b) = b.as_ref() {
-            self.resolve_expression(b)?;
-          }
-          Ok(())
-        }
-        Thunk::UnaryPlus {
-          args: (a, rest), ..
-        } => {
-          self.resolve_expression(a)?;
-          for arg in rest {
-            self.resolve_expression(arg)?;
-          }
-          Ok(())
-        }
-        Thunk::Binary { args: [a, b], .. } => {
-          self.resolve_expression(a)?;
-          self.resolve_expression(b)
-        }
-        Thunk::BinaryPlus {
-          args: ([a, b], rest),
-          ..
-        } => {
-          self.resolve_expression(a)?;
-          self.resolve_expression(b)?;
-          for arg in rest {
-            self.resolve_expression(arg)?;
-          }
-          Ok(())
-        }
-        Thunk::Ternary {
-          args: [a, b, c], ..
-        } => {
-          self.resolve_expression(a)?;
-          self.resolve_expression(b)?;
-          self.resolve_expression(c)
-        }
-      },
-      Expression::Concatenation { lhs, rhs } => {
-        self.resolve_expression(lhs)?;
-        self.resolve_expression(rhs)
-      }
-      Expression::Conditional {
-        condition: Condition {
-          lhs,
-          rhs,
-          operator: _,
-        },
-        then,
-        otherwise,
-        ..
-      } => {
-        self.resolve_expression(lhs)?;
-        self.resolve_expression(rhs)?;
-        self.resolve_expression(then)?;
-        self.resolve_expression(otherwise)
-      }
-      Expression::Group { contents } => self.resolve_expression(contents),
-      Expression::Join { lhs, rhs } => {
-        if let Some(lhs) = lhs {
-          self.resolve_expression(lhs)?;
-        }
-        self.resolve_expression(rhs)
-      }
-      Expression::StringLiteral { .. } | Expression::Backtick { .. } => Ok(()),
-      Expression::Variable { name } => {
-        let variable = name.lexeme();
-        if self.evaluated.contains(variable) || constants().contains_key(variable) {
-          Ok(())
-        } else if self.stack.contains(&variable) {
-          self.stack.push(variable);
-          Err(
-            self.assignments[variable]
-              .name
-              .error(CircularVariableDependency {
-                variable,
-                circle: self.stack.clone(),
-              }),
-          )
-        } else if self.assignments.contains_key(variable) {
-          self.resolve_assignment(variable)
-        } else {
-          Err(name.token.error(UndefinedVariable { variable }))
-        }
-      }
-    }
-  }
 }
 
 #[cfg(test)]

src/ast.rs

@@ -6,6 +6,7 @@ use super::*;
 #[derive(Debug, Clone)]
 pub(crate) struct Ast<'src> {
   pub(crate) items: Vec<Item<'src>>,
+  pub(crate) unstable_features: BTreeSet<UnstableFeature>,
   pub(crate) warnings: Vec<Warning>,
   pub(crate) working_directory: PathBuf,
 }

src/evaluator.rs

@@ -84,24 +84,31 @@ impl<'src, 'run> Evaluator<'src, 'run> {
     expression: &Expression<'src>,
   ) -> RunResult<'src, String> {
     match expression {
-      Expression::Variable { name, .. } => {
-        let variable = name.lexeme();
-        if let Some(value) = self.scope.value(variable) {
-          Ok(value.to_owned())
-        } else if let Some(assignment) = self
-          .assignments
-          .and_then(|assignments| assignments.get(variable))
-        {
-          Ok(self.evaluate_assignment(assignment)?.to_owned())
+      Expression::And { lhs, rhs } => {
+        let lhs = self.evaluate_expression(lhs)?;
+        if lhs.is_empty() {
+          return Ok(String::new());
+        }
+        self.evaluate_expression(rhs)
+      }
+      Expression::Assert { condition, error } => {
+        if self.evaluate_condition(condition)? {
+          Ok(String::new())
         } else {
-          Err(Error::Internal {
-            message: format!("attempted to evaluate undefined variable `{variable}`"),
+          Err(Error::Assert {
+            message: self.evaluate_expression(error)?,
           })
         }
       }
+      Expression::Backtick { contents, token } => {
+        if self.context.config.dry_run {
+          Ok(format!("`{contents}`"))
+        } else {
+          Ok(self.run_backtick(contents, token)?)
+        }
+      }
       Expression::Call { thunk } => {
         use Thunk::*;
-
         let result = match thunk {
           Nullary { function, .. } => function(function::Context::new(self, thunk.name())),
           Unary { function, arg, .. } => {
@@ -118,7 +125,6 @@ impl<'src, 'run> Evaluator<'src, 'run> {
               Some(b) => Some(self.evaluate_expression(b)?),
               None => None,
             };
-
             function(function::Context::new(self, thunk.name()), &a, b.as_deref())
           }
           UnaryPlus {
@@ -175,20 +181,11 @@ impl<'src, 'run> Evaluator<'src, 'run> {
             function(function::Context::new(self, thunk.name()), &a, &b, &c)
           }
         };
-
         result.map_err(|message| Error::FunctionCall {
           function: thunk.name(),
           message,
         })
       }
-      Expression::StringLiteral { string_literal } => Ok(string_literal.cooked.clone()),
-      Expression::Backtick { contents, token } => {
-        if self.context.config.dry_run {
-          Ok(format!("`{contents}`"))
-        } else {
-          Ok(self.run_backtick(contents, token)?)
-        }
-      }
       Expression::Concatenation { lhs, rhs } => {
         Ok(self.evaluate_expression(lhs)? + &self.evaluate_expression(rhs)?)
       }
@@ -209,12 +206,26 @@ impl<'src, 'run> Evaluator<'src, 'run> {
         lhs: Some(lhs),
         rhs,
       } => Ok(self.evaluate_expression(lhs)? + "/" + &self.evaluate_expression(rhs)?),
-      Expression::Assert { condition, error } => {
-        if self.evaluate_condition(condition)? {
-          Ok(String::new())
+      Expression::Or { lhs, rhs } => {
+        let lhs = self.evaluate_expression(lhs)?;
+        if !lhs.is_empty() {
+          return Ok(lhs);
+        }
+        self.evaluate_expression(rhs)
+      }
+      Expression::StringLiteral { string_literal } => Ok(string_literal.cooked.clone()),
+      Expression::Variable { name, .. } => {
+        let variable = name.lexeme();
+        if let Some(value) = self.scope.value(variable) {
+          Ok(value.to_owned())
+        } else if let Some(assignment) = self
+          .assignments
+          .and_then(|assignments| assignments.get(variable))
+        {
+          Ok(self.evaluate_assignment(assignment)?.to_owned())
         } else {
-          Err(Error::Assert {
-            message: self.evaluate_expression(error)?,
+          Err(Error::Internal {
+            message: format!("attempted to evaluate undefined variable `{variable}`"),
           })
         }
       }

src/expression.rs

@@ -8,6 +8,11 @@ use super::*;
 /// The parser parses both values and expressions into `Expression`s.
 #[derive(PartialEq, Debug, Clone)]
 pub(crate) enum Expression<'src> {
+  /// `lhs && rhs`
+  And {
+    lhs: Box<Expression<'src>>,
+    rhs: Box<Expression<'src>>,
+  },
   /// `assert(condition, error)`
   Assert {
     condition: Condition<'src>,
@@ -38,6 +43,11 @@ pub(crate) enum Expression<'src> {
     lhs: Option<Box<Expression<'src>>>,
     rhs: Box<Expression<'src>>,
   },
+  /// `lhs || rhs`
+  Or {
+    lhs: Box<Expression<'src>>,
+    rhs: Box<Expression<'src>>,
+  },
   /// `"string_literal"` or `'string_literal'`
   StringLiteral { string_literal: StringLiteral<'src> },
   /// `variable`
@@ -53,23 +63,25 @@ impl<'src> Expression<'src> {
 impl<'src> Display for Expression<'src> {
   fn fmt(&self, f: &mut Formatter) -> fmt::Result {
     match self {
+      Self::And { lhs, rhs } => write!(f, "{lhs} && {rhs}"),
       Self::Assert { condition, error } => write!(f, "assert({condition}, {error})"),
       Self::Backtick { token, .. } => write!(f, "{}", token.lexeme()),
-      Self::Join { lhs: None, rhs } => write!(f, "/ {rhs}"),
-      Self::Join {
-        lhs: Some(lhs),
-        rhs,
-      } => write!(f, "{lhs} / {rhs}"),
+      Self::Call { thunk } => write!(f, "{thunk}"),
       Self::Concatenation { lhs, rhs } => write!(f, "{lhs} + {rhs}"),
       Self::Conditional {
         condition,
         then,
         otherwise,
       } => write!(f, "if {condition} {{ {then} }} else {{ {otherwise} }}"),
+      Self::Group { contents } => write!(f, "({contents})"),
+      Self::Join { lhs: None, rhs } => write!(f, "/ {rhs}"),
+      Self::Join {
+        lhs: Some(lhs),
+        rhs,
+      } => write!(f, "{lhs} / {rhs}"),
+      Self::Or { lhs, rhs } => write!(f, "{lhs} || {rhs}"),
       Self::StringLiteral { string_literal } => write!(f, "{string_literal}"),
       Self::Variable { name } => write!(f, "{}", name.lexeme()),
-      Self::Call { thunk } => write!(f, "{thunk}"),
-      Self::Group { contents } => write!(f, "({contents})"),
     }
   }
 }
@@ -80,6 +92,13 @@ impl<'src> Serialize for Expression<'src> {
     S: Serializer,
   {
     match self {
+      Self::And { lhs, rhs } => {
+        let mut seq = serializer.serialize_seq(None)?;
+        seq.serialize_element("and")?;
+        seq.serialize_element(lhs)?;
+        seq.serialize_element(rhs)?;
+        seq.end()
+      }
       Self::Assert { condition, error } => {
         let mut seq: <S as Serializer>::SerializeSeq = serializer.serialize_seq(None)?;
         seq.serialize_element("assert")?;
@@ -101,13 +120,6 @@ impl<'src> Serialize for Expression<'src> {
         seq.serialize_element(rhs)?;
         seq.end()
       }
-      Self::Join { lhs, rhs } => {
-        let mut seq = serializer.serialize_seq(None)?;
-        seq.serialize_element("join")?;
-        seq.serialize_element(lhs)?;
-        seq.serialize_element(rhs)?;
-        seq.end()
-      }
       Self::Conditional {
         condition,
         then,
@@ -121,6 +133,20 @@ impl<'src> Serialize for Expression<'src> {
         seq.end()
       }
       Self::Group { contents } => contents.serialize(serializer),
+      Self::Join { lhs, rhs } => {
+        let mut seq = serializer.serialize_seq(None)?;
+        seq.serialize_element("join")?;
+        seq.serialize_element(lhs)?;
+        seq.serialize_element(rhs)?;
+        seq.end()
+      }
+      Self::Or { lhs, rhs } => {
+        let mut seq = serializer.serialize_seq(None)?;
+        seq.serialize_element("or")?;
+        seq.serialize_element(lhs)?;
+        seq.serialize_element(rhs)?;
+        seq.end()
+      }
       Self::StringLiteral { string_literal } => string_literal.serialize(serializer),
       Self::Variable { name } => {
         let mut seq = serializer.serialize_seq(None)?;

src/lexer.rs

@@ -496,6 +496,7 @@ impl<'src> Lexer<'src> {
       ']' => self.lex_delimiter(BracketR),
       '`' | '"' | '\'' => self.lex_string(),
       '{' => self.lex_delimiter(BraceL),
+      '|' => self.lex_digraph('|', '|', BarBar),
       '}' => self.lex_delimiter(BraceR),
       _ if Self::is_identifier_start(start) => self.lex_identifier(),
       _ => {
@@ -948,6 +949,7 @@ mod tests {
       Asterisk => "*",
       At => "@",
       BangEquals => "!=",
+      BarBar => "||",
       BraceL => "{",
       BraceR => "}",
       BracketL => "[",

src/node.rs

@@ -88,6 +88,7 @@ impl<'src> Node<'src> for Assignment<'src> {
 impl<'src> Node<'src> for Expression<'src> {
   fn tree(&self) -> Tree<'src> {
     match self {
+      Self::And { lhs, rhs } => Tree::atom("&&").push(lhs.tree()).push(rhs.tree()),
       Self::Assert {
         condition: Condition { lhs, rhs, operator },
         error,
@@ -96,25 +97,10 @@ impl<'src> Node<'src> for Expression<'src> {
         .push(operator.to_string())
         .push(rhs.tree())
         .push(error.tree()),
-      Self::Concatenation { lhs, rhs } => Tree::atom("+").push(lhs.tree()).push(rhs.tree()),
-      Self::Conditional {
-        condition: Condition { lhs, rhs, operator },
-        then,
-        otherwise,
-      } => {
-        let mut tree = Tree::atom(Keyword::If.lexeme());
-        tree.push_mut(lhs.tree());
-        tree.push_mut(operator.to_string());
-        tree.push_mut(rhs.tree());
-        tree.push_mut(then.tree());
-        tree.push_mut(otherwise.tree());
-        tree
-      }
+      Self::Backtick { contents, .. } => Tree::atom("backtick").push(Tree::string(contents)),
       Self::Call { thunk } => {
         use Thunk::*;
-
         let mut tree = Tree::atom("call");
-
         match thunk {
           Nullary { name, .. } => tree.push_mut(name.lexeme()),
           Unary { name, arg, .. } => {
@@ -171,20 +157,33 @@ impl<'src> Node<'src> for Expression<'src> {
             tree.push_mut(c.tree());
           }
         }
-
         tree
       }
-      Self::Variable { name } => Tree::atom(name.lexeme()),
-      Self::StringLiteral {
-        string_literal: StringLiteral { cooked, .. },
-      } => Tree::string(cooked),
-      Self::Backtick { contents, .. } => Tree::atom("backtick").push(Tree::string(contents)),
+      Self::Concatenation { lhs, rhs } => Tree::atom("+").push(lhs.tree()).push(rhs.tree()),
+      Self::Conditional {
+        condition: Condition { lhs, rhs, operator },
+        then,
+        otherwise,
+      } => {
+        let mut tree = Tree::atom(Keyword::If.lexeme());
+        tree.push_mut(lhs.tree());
+        tree.push_mut(operator.to_string());
+        tree.push_mut(rhs.tree());
+        tree.push_mut(then.tree());
+        tree.push_mut(otherwise.tree());
+        tree
+      }
       Self::Group { contents } => Tree::List(vec![contents.tree()]),
       Self::Join { lhs: None, rhs } => Tree::atom("/").push(rhs.tree()),
       Self::Join {
         lhs: Some(lhs),
         rhs,
       } => Tree::atom("/").push(lhs.tree()).push(rhs.tree()),
+      Self::Or { lhs, rhs } => Tree::atom("||").push(lhs.tree()).push(rhs.tree()),
+      Self::StringLiteral {
+        string_literal: StringLiteral { cooked, .. },
+      } => Tree::string(cooked),
+      Self::Variable { name } => Tree::atom(name.lexeme()),
     }
   }
 }

src/parser.rs

@@ -31,6 +31,7 @@ pub(crate) struct Parser<'run, 'src> {
   next_token: usize,
   recursion_depth: usize,
   tokens: &'run [Token<'src>],
+  unstable_features: BTreeSet<UnstableFeature>,
   working_directory: &'run Path,
 }
 
@@ -51,6 +52,7 @@ impl<'run, 'src> Parser<'run, 'src> {
       next_token: 0,
       recursion_depth: 0,
       tokens,
+      unstable_features: BTreeSet::new(),
       working_directory,
     }
     .parse_ast()
@@ -442,18 +444,19 @@ impl<'run, 'src> Parser<'run, 'src> {
       }
     }
 
-    if self.next_token == self.tokens.len() {
-      Ok(Ast {
-        items,
-        warnings: Vec::new(),
-        working_directory: self.working_directory.into(),
-      })
-    } else {
-      Err(self.internal_error(format!(
+    if self.next_token != self.tokens.len() {
+      return Err(self.internal_error(format!(
         "Parse completed with {} unparsed tokens",
         self.tokens.len() - self.next_token,
-      ))?)
+      ))?);
     }
+
+    Ok(Ast {
+      items,
+      unstable_features: self.unstable_features,
+      warnings: Vec::new(),
+      working_directory: self.working_directory.into(),
+    })
   }
 
   /// Parse an alias, e.g `alias name := target`
@@ -517,26 +520,17 @@ impl<'run, 'src> Parser<'run, 'src> {
 
     self.recursion_depth += 1;
 
-    let expression = if self.accepted_keyword(Keyword::If)? {
-      self.parse_conditional()?
-    } else if self.accepted(Slash)? {
-      let lhs = None;
-      let rhs = self.parse_expression()?.into();
-      Expression::Join { lhs, rhs }
-    } else {
-      let value = self.parse_value()?;
+    let disjunct = self.parse_disjunct()?;
 
-      if self.accepted(Slash)? {
-        let lhs = Some(Box::new(value));
-        let rhs = self.parse_expression()?.into();
-        Expression::Join { lhs, rhs }
-      } else if self.accepted(Plus)? {
-        let lhs = value.into();
-        let rhs = self.parse_expression()?.into();
-        Expression::Concatenation { lhs, rhs }
-      } else {
-        value
-      }
+    let expression = if self.accepted(BarBar)? {
+      self
+        .unstable_features
+        .insert(UnstableFeature::LogicalOperators);
+      let lhs = disjunct.into();
+      let rhs = self.parse_expression()?.into();
+      Expression::Or { lhs, rhs }
+    } else {
+      disjunct
     };
 
     self.recursion_depth -= 1;
@@ -544,6 +538,47 @@ impl<'run, 'src> Parser<'run, 'src> {
     Ok(expression)
   }
 
+  fn parse_disjunct(&mut self) -> CompileResult<'src, Expression<'src>> {
+    let conjunct = self.parse_conjunct()?;
+
+    let disjunct = if self.accepted(AmpersandAmpersand)? {
+      self
+        .unstable_features
+        .insert(UnstableFeature::LogicalOperators);
+      let lhs = conjunct.into();
+      let rhs = self.parse_disjunct()?.into();
+      Expression::And { lhs, rhs }
+    } else {
+      conjunct
+    };
+
+    Ok(disjunct)
+  }
+
+  fn parse_conjunct(&mut self) -> CompileResult<'src, Expression<'src>> {
+    if self.accepted_keyword(Keyword::If)? {
+      self.parse_conditional()
+    } else if self.accepted(Slash)? {
+      let lhs = None;
+      let rhs = self.parse_conjunct()?.into();
+      Ok(Expression::Join { lhs, rhs })
+    } else {
+      let value = self.parse_value()?;
+
+      if self.accepted(Slash)? {
+        let lhs = Some(Box::new(value));
+        let rhs = self.parse_conjunct()?.into();
+        Ok(Expression::Join { lhs, rhs })
+      } else if self.accepted(Plus)? {
+        let lhs = value.into();
+        let rhs = self.parse_conjunct()?.into();
+        Ok(Expression::Concatenation { lhs, rhs })
+      } else {
+        Ok(value)
+      }
+    }
+  }
+
   /// Parse a conditional, e.g. `if a == b { "foo" } else { "bar" }`
   fn parse_conditional(&mut self) -> CompileResult<'src, Expression<'src>> {
     let condition = self.parse_condition()?;

src/summary.rs

@@ -183,6 +183,10 @@ impl Assignment {
 
 #[derive(Eq, PartialEq, Hash, Ord, PartialOrd, Debug, Clone)]
 pub enum Expression {
+  And {
+    lhs: Box<Expression>,
+    rhs: Box<Expression>,
+  },
   Assert {
     condition: Condition,
     error: Box<Expression>,
@@ -209,6 +213,10 @@ pub enum Expression {
     lhs: Option<Box<Expression>>,
     rhs: Box<Expression>,
   },
+  Or {
+    lhs: Box<Expression>,
+    rhs: Box<Expression>,
+  },
   String {
     text: String,
   },
@@ -221,6 +229,10 @@ impl Expression {
   fn new(expression: &full::Expression) -> Self {
     use full::Expression::*;
     match expression {
+      And { lhs, rhs } => Self::And {
+        lhs: Self::new(lhs).into(),
+        rhs: Self::new(rhs).into(),
+      },
       Assert {
         condition: full::Condition { lhs, rhs, operator },
         error,
@@ -250,11 +262,9 @@ impl Expression {
           ..
         } => {
           let mut arguments = Vec::new();
-
           if let Some(b) = opt_b.as_ref() {
             arguments.push(Self::new(b));
           }
-
           arguments.push(Self::new(a));
           Self::Call {
             name: name.lexeme().to_owned(),
@@ -308,10 +318,6 @@ impl Expression {
         lhs: Self::new(lhs).into(),
         rhs: Self::new(rhs).into(),
       },
-      Join { lhs, rhs } => Self::Join {
-        lhs: lhs.as_ref().map(|lhs| Self::new(lhs).into()),
-        rhs: Self::new(rhs).into(),
-      },
       Conditional {
         condition: full::Condition { lhs, rhs, operator },
         otherwise,
@@ -323,13 +329,21 @@ impl Expression {
         rhs: Self::new(rhs).into(),
         then: Self::new(then).into(),
       },
+      Group { contents } => Self::new(contents),
+      Join { lhs, rhs } => Self::Join {
+        lhs: lhs.as_ref().map(|lhs| Self::new(lhs).into()),
+        rhs: Self::new(rhs).into(),
+      },
+      Or { lhs, rhs } => Self::Or {
+        lhs: Self::new(lhs).into(),
+        rhs: Self::new(rhs).into(),
+      },
       StringLiteral { string_literal } => Self::String {
         text: string_literal.cooked.clone(),
       },
       Variable { name, .. } => Self::Variable {
         name: name.lexeme().to_owned(),
       },
-      Group { contents } => Self::new(contents),
     }
   }
 }

src/token_kind.rs

@@ -7,6 +7,7 @@ pub(crate) enum TokenKind {
   At,
   Backtick,
   BangEquals,
+  BarBar,
   BraceL,
   BraceR,
   BracketL,
@@ -50,6 +51,7 @@ impl Display for TokenKind {
         At => "'@'",
         Backtick => "backtick",
         BangEquals => "'!='",
+        BarBar => "'||'",
         BraceL => "'{'",
         BraceR => "'}'",
         BracketL => "'['",

src/unstable_feature.rs

@@ -3,6 +3,7 @@ use super::*;
 #[derive(Copy, Clone, Debug, PartialEq, Ord, Eq, PartialOrd)]
 pub(crate) enum UnstableFeature {
   FormatSubcommand,
+  LogicalOperators,
   ScriptAttribute,
   ScriptInterpreterSetting,
 }
@@ -11,6 +12,10 @@ impl Display for UnstableFeature {
   fn fmt(&self, f: &mut Formatter) -> fmt::Result {
     match self {
       Self::FormatSubcommand => write!(f, "The `--fmt` command is currently unstable."),
+      Self::LogicalOperators => write!(
+        f,
+        "The logical operators `&&` and `||` are currently unstable."
+      ),
       Self::ScriptAttribute => write!(f, "The `[script]` attribute is currently unstable."),
       Self::ScriptInterpreterSetting => {
         write!(f, "The `script-interpreter` setting is currently unstable.")

src/variables.rs

@@ -16,7 +16,24 @@ impl<'expression, 'src> Iterator for Variables<'expression, 'src> {
   fn next(&mut self) -> Option<Token<'src>> {
     loop {
       match self.stack.pop()? {
-        Expression::StringLiteral { .. } | Expression::Backtick { .. } => {}
+        Expression::And { lhs, rhs } | Expression::Or { lhs, rhs } => {
+          self.stack.push(lhs);
+          self.stack.push(rhs);
+        }
+        Expression::Assert {
+          condition:
+            Condition {
+              lhs,
+              rhs,
+              operator: _,
+            },
+          error,
+        } => {
+          self.stack.push(error);
+          self.stack.push(rhs);
+          self.stack.push(lhs);
+        }
+        Expression::Backtick { .. } | Expression::StringLiteral { .. } => {}
         Expression::Call { thunk } => match thunk {
           Thunk::Nullary { .. } => {}
           Thunk::Unary { arg, .. } => self.stack.push(arg),
@@ -56,6 +73,10 @@ impl<'expression, 'src> Iterator for Variables<'expression, 'src> {
             }
           }
         },
+        Expression::Concatenation { lhs, rhs } => {
+          self.stack.push(rhs);
+          self.stack.push(lhs);
+        }
         Expression::Conditional {
           condition:
             Condition {
@@ -71,10 +92,8 @@ impl<'expression, 'src> Iterator for Variables<'expression, 'src> {
           self.stack.push(rhs);
           self.stack.push(lhs);
         }
-        Expression::Variable { name, .. } => return Some(name.token),
-        Expression::Concatenation { lhs, rhs } => {
-          self.stack.push(rhs);
-          self.stack.push(lhs);
+        Expression::Group { contents } => {
+          self.stack.push(contents);
         }
         Expression::Join { lhs, rhs } => {
           self.stack.push(rhs);
@@ -82,22 +101,7 @@ impl<'expression, 'src> Iterator for Variables<'expression, 'src> {
             self.stack.push(lhs);
           }
         }
-        Expression::Group { contents } => {
-          self.stack.push(contents);
-        }
-        Expression::Assert {
-          condition:
-            Condition {
-              lhs,
-              rhs,
-              operator: _,
-            },
-          error,
-        } => {
-          self.stack.push(error);
-          self.stack.push(rhs);
-          self.stack.push(lhs);
-        }
+        Expression::Variable { name, .. } => return Some(name.token),
       }
     }
   }

tests/assert_success.rs

@@ -1,3 +1,4 @@
+#[track_caller]
 pub(crate) fn assert_success(output: &std::process::Output) {
   if !output.status.success() {
     eprintln!("stderr: {}", String::from_utf8_lossy(&output.stderr));

tests/conditional.rs

@@ -136,7 +136,7 @@ test! {
   ",
   stdout: "",
   stderr: "
-    error: Expected '!=', '==', '=~', '+', or '/', but found identifier
+    error: Expected '&&', '!=', '||', '==', '=~', '+', or '/', but found identifier
      ——▶ justfile:1:12
       │
     1 │ a := if '' a '' { '' } else { b }

tests/ignore_comments.rs

@@ -125,7 +125,7 @@ fn comments_still_must_be_parsable_when_ignored() {
     )
     .stderr(
       "
-        error: Expected '}}', '(', '+', or '/', but found identifier
+        error: Expected '&&', '||', '}}', '(', '+', or '/', but found identifier
          ——▶ justfile:4:12
           │
         4 │   # {{ foo bar }}

tests/lib.rs

@@ -73,6 +73,7 @@ mod invocation_directory;
 mod json;
 mod line_prefixes;
 mod list;
+mod logical_operators;
 mod man;
 mod misc;
 mod modules;

tests/logical_operators.rs

@@ -0,0 +1,83 @@
+use super::*;
+
+#[track_caller]
+fn evaluate(expression: &str, expected: &str) {
+  Test::new()
+    .justfile(format!("x := {expression}"))
+    .env("JUST_UNSTABLE", "1")
+    .args(["--evaluate", "x"])
+    .stdout(expected)
+    .run();
+}
+
+#[test]
+fn logical_operators_are_unstable() {
+  Test::new()
+    .justfile("x := 'foo' && 'bar'")
+    .args(["--evaluate", "x"])
+    .stderr_regex(r"error: The logical operators `&&` and `\|\|` are currently unstable. .*")
+    .status(EXIT_FAILURE)
+    .run();
+
+  Test::new()
+    .justfile("x := 'foo' || 'bar'")
+    .args(["--evaluate", "x"])
+    .stderr_regex(r"error: The logical operators `&&` and `\|\|` are currently unstable. .*")
+    .status(EXIT_FAILURE)
+    .run();
+}
+
+#[test]
+fn and_returns_empty_string_if_lhs_is_empty() {
+  evaluate("'' && 'hello'", "");
+}
+
+#[test]
+fn and_returns_rhs_if_lhs_is_non_empty() {
+  evaluate("'hello' && 'goodbye'", "goodbye");
+}
+
+#[test]
+fn and_has_lower_precedence_than_plus() {
+  evaluate("'' && 'goodbye' + 'foo'", "");
+
+  evaluate("'foo' + 'hello' && 'goodbye'", "goodbye");
+
+  evaluate("'foo' + '' && 'goodbye'", "goodbye");
+
+  evaluate("'foo' + 'hello' && 'goodbye' + 'bar'", "goodbyebar");
+}
+
+#[test]
+fn or_returns_rhs_if_lhs_is_empty() {
+  evaluate("'' || 'hello'", "hello");
+}
+
+#[test]
+fn or_returns_lhs_if_lhs_is_non_empty() {
+  evaluate("'hello' || 'goodbye'", "hello");
+}
+
+#[test]
+fn or_has_lower_precedence_than_plus() {
+  evaluate("'' || 'goodbye' + 'foo'", "goodbyefoo");
+
+  evaluate("'foo' + 'hello' || 'goodbye'", "foohello");
+
+  evaluate("'foo' + '' || 'goodbye'", "foo");
+
+  evaluate("'foo' + 'hello' || 'goodbye' + 'bar'", "foohello");
+}
+
+#[test]
+fn and_has_higher_precedence_than_or() {
+  evaluate("('' && 'foo') || 'bar'", "bar");
+  evaluate("'' && 'foo' || 'bar'", "bar");
+  evaluate("'a' && 'b' || 'c'", "b");
+}
+
+#[test]
+fn nesting() {
+  evaluate("'' || '' || '' || '' || 'foo'", "foo");
+  evaluate("'foo' && 'foo' && 'foo' && 'foo' && 'bar'", "bar");
+}

tests/shell_expansion.rs

@@ -25,7 +25,7 @@ fn shell_expanded_strings_must_not_have_whitespace() {
     .status(1)
     .stderr(
       "
-        error: Expected comment, end of file, end of line, '(', '+', or '/', but found string
+        error: Expected '&&', '||', comment, end of file, end of line, '(', '+', or '/', but found string
          ——▶ justfile:1:8
           │
         1 │ x := x '$JUST_TEST_VARIABLE'