Add && and || operators

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 `{{…}}` substitutions
+inside recipe bodies:
 
 ```just
 tmpdir  := `mktemp -d`
@@ -1310,6 +1312,33 @@ publish:
   rm -rf {{tarball}} {{tardir}}
 ```
 
+#### Concatenation
+
+The `+` operator returns the left-hand argument concatenated with the
+right-hand argument:
+
+```just
+foobar := 'foo' + 'bar'
+```
+
+#### `&&` and/or `||`
+
+The `&&` operator returns the empty string if the left-hand argument is the
+empty string, otherwise it returns the right hand argument:
+
+```just
+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:
+
+```just
+foo := '' || 'goodbye'      # 'goodbye'
+bar := 'hello' || 'goodbye' # 'hello'
+```
+
 #### Joining Paths
 
 The `/` operator can be used to join two strings with a slash:
@@ -2367,8 +2396,8 @@ 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"

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/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(lhs);
+        }
+        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}`"),
           })
         }
       }