Add elvis operator

docs/content/main/examples.md

@@ -1,7 +1,7 @@
 +++
 title = "Examples"
 slug = "Sample configs"
-weight = 5
+weight = 6
 +++
 
 ## Example Configurations

docs/content/main/expression_language.md

@@ -1,7 +1,7 @@
 +++
 title = "Eww expressions"
 slug = "Embedded eww expression language"
-weight = 6
+weight = 5
 +++
 
 # The embedded Eww expression-language
@@ -27,6 +27,8 @@ The expression language supports:
 - simple mathematical operations (`+`, `-`, `*`, `/`, `%`)
 - comparisons (`==`, `!=`, `>`, `<`)
 - boolean operations (`||`, `&&`, `!`)
+- elvis operator (`?:`)
+    - if the left side is `""`, then returns the right side, otherwise evaluates to the left side.
 - conditionals (`if condition then 'value' else 'other value'`)
 - numbers, strings, booleans and variable references (`12`, `'hi'`, `true`, `some_variable`)
     - strings can contain other expressions again: `'foo {{some_variable}} bar'`

src/value/attr_value/attr_value_expr.rs

@@ -16,6 +16,7 @@ pub enum BinOp {
     Or,
     GT,
     LT,
+    Elvis,
 }
 
 impl std::fmt::Display for BinOp {
@@ -32,6 +33,7 @@ impl std::fmt::Display for BinOp {
             BinOp::Or => write!(f, "||"),
             BinOp::GT => write!(f, ">"),
             BinOp::LT => write!(f, "<"),
+            BinOp::Elvis => write!(f, "?:"),
         }
     }
 }
@@ -150,6 +152,7 @@ impl AttrValueExpr {
                     BinOp::Mod => PrimitiveValue::from(a.as_f64()? % b.as_f64()?),
                     BinOp::GT => PrimitiveValue::from(a.as_f64()? > b.as_f64()?),
                     BinOp::LT => PrimitiveValue::from(a.as_f64()? < b.as_f64()?),
+                    BinOp::Elvis => PrimitiveValue::from(if a.0.is_empty() { b } else { a }),
                 })
             }
             AttrValueExpr::UnaryOp(op, a) => {

src/value/attr_value/parser.rs

@@ -131,6 +131,7 @@ pub fn parse_expr(i: &str) -> IResult<&str, AttrValueExpr, VerboseError<&str>> {
     let (i, remainder) = many0(alt((
         map(preceded(tag("&&"), parse_term1), |x| (BinOp::And, x)),
         map(preceded(tag("||"), parse_term1), |x| (BinOp::Or, x)),
+        map(preceded(tag("?:"), parse_term1), |x| (BinOp::Elvis, x)),
     )))(i)?;
 
     let exprs = remainder

src/value/primitive.rs

@@ -6,7 +6,7 @@ use std::{convert::TryFrom, fmt, iter::FromIterator};
 use crate::impl_try_from;
 
 #[derive(Clone, Deserialize, Serialize, derive_more::From, Default)]
-pub struct PrimitiveValue(String);
+pub struct PrimitiveValue(pub String);
 
 impl fmt::Display for PrimitiveValue {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {