Safe Access Operator (#577)

Co-authored-by: elkowar <5300871+elkowar@users.noreply.github.com>

CHANGELOG.md

@@ -5,6 +5,9 @@ All notable changes to eww will be listed here, starting at changes since versio
 
 ## [Unreleased]
 
+### Features
+- Add support for safe access (`?.`) in simplexpr (By: oldwomanjosiah)
+
 ## [0.4.0] (04.09.2022)
 
 ### BREAKING CHANGES

crates/simplexpr/src/ast.rs

@@ -33,6 +33,13 @@ pub enum UnaryOp {
     Negative,
 }
 
+/// Differenciates between regular field access (`foo.bar`) and null-safe field access (`foo?.bar`)
+#[derive(Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum AccessType {
+    Normal,
+    Safe,
+}
+
 #[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub enum SimplExpr {
     Literal(DynVal),
@@ -43,7 +50,7 @@ pub enum SimplExpr {
     BinOp(Span, Box<SimplExpr>, BinOp, Box<SimplExpr>),
     UnaryOp(Span, UnaryOp, Box<SimplExpr>),
     IfElse(Span, Box<SimplExpr>, Box<SimplExpr>, Box<SimplExpr>),
-    JsonAccess(Span, Box<SimplExpr>, Box<SimplExpr>),
+    JsonAccess(Span, AccessType, Box<SimplExpr>, Box<SimplExpr>),
     FunctionCall(Span, String, Vec<SimplExpr>),
 }
 
@@ -65,7 +72,8 @@ impl std::fmt::Display for SimplExpr {
             SimplExpr::BinOp(_, l, op, r) => write!(f, "({} {} {})", l, op, r),
             SimplExpr::UnaryOp(_, op, x) => write!(f, "{}{}", op, x),
             SimplExpr::IfElse(_, a, b, c) => write!(f, "({} ? {} : {})", a, b, c),
-            SimplExpr::JsonAccess(_, value, index) => write!(f, "{}[{}]", value, index),
+            SimplExpr::JsonAccess(_, AccessType::Normal, value, index) => write!(f, "{}[{}]", value, index),
+            SimplExpr::JsonAccess(_, AccessType::Safe, value, index) => write!(f, "{}?.[{}]", value, index),
             SimplExpr::FunctionCall(_, function_name, args) => {
                 write!(f, "{}({})", function_name, args.iter().join(", "))
             }
@@ -101,7 +109,7 @@ impl SimplExpr {
             Literal(_) => false,
             Concat(_, x) | FunctionCall(_, _, x) | JsonArray(_, x) => x.iter().any(|x| x.references_var(var)),
             JsonObject(_, x) => x.iter().any(|(k, v)| k.references_var(var) || v.references_var(var)),
-            JsonAccess(_, a, b) | BinOp(_, a, _, b) => a.references_var(var) || b.references_var(var),
+            JsonAccess(_, _, a, b) | BinOp(_, a, _, b) => a.references_var(var) || b.references_var(var),
             UnaryOp(_, _, x) => x.references_var(var),
             IfElse(_, a, b, c) => a.references_var(var) || b.references_var(var) || c.references_var(var),
             VarRef(_, x) => x == var,
@@ -113,7 +121,7 @@ impl SimplExpr {
         match self {
             VarRef(_, x) => dest.push(x.clone()),
             UnaryOp(_, _, x) => x.as_ref().collect_var_refs_into(dest),
-            BinOp(_, a, _, b) | JsonAccess(_, a, b) => {
+            BinOp(_, a, _, b) | JsonAccess(_, _, a, b) => {
                 a.as_ref().collect_var_refs_into(dest);
                 b.as_ref().collect_var_refs_into(dest);
             }

crates/simplexpr/src/eval.rs

@@ -1,7 +1,7 @@
 use itertools::Itertools;
 
 use crate::{
-    ast::{BinOp, SimplExpr, UnaryOp},
+    ast::{AccessType, BinOp, SimplExpr, UnaryOp},
     dynval::{ConversionError, DynVal},
 };
 use eww_shared_util::{Span, Spanned, VarName};
@@ -75,7 +75,9 @@ impl SimplExpr {
             IfElse(span, box a, box b, box c) => {
                 IfElse(span, box a.try_map_var_refs(f)?, box b.try_map_var_refs(f)?, box c.try_map_var_refs(f)?)
             }
-            JsonAccess(span, box a, box b) => JsonAccess(span, box a.try_map_var_refs(f)?, box b.try_map_var_refs(f)?),
+            JsonAccess(span, safe, box a, box b) => {
+                JsonAccess(span, safe, box a.try_map_var_refs(f)?, box b.try_map_var_refs(f)?)
+            }
             FunctionCall(span, name, args) => {
                 FunctionCall(span, name, args.into_iter().map(|x| x.try_map_var_refs(f)).collect::<Result<_, _>>()?)
             }
@@ -124,7 +126,7 @@ impl SimplExpr {
             Literal(..) => Vec::new(),
             VarRef(span, name) => vec![(*span, name)],
             Concat(_, elems) => elems.iter().flat_map(|x| x.var_refs_with_span().into_iter()).collect(),
-            BinOp(_, box a, _, box b) | JsonAccess(_, box a, box b) => {
+            BinOp(_, box a, _, box b) | JsonAccess(_, _, box a, box b) => {
                 let mut refs = a.var_refs_with_span();
                 refs.extend(b.var_refs_with_span().iter());
                 refs
@@ -195,8 +197,14 @@ impl SimplExpr {
                     BinOp::LT => DynVal::from(a.as_f64()? < b.as_f64()?),
                     BinOp::GE => DynVal::from(a.as_f64()? >= b.as_f64()?),
                     BinOp::LE => DynVal::from(a.as_f64()? <= b.as_f64()?),
-                    #[allow(clippy::useless_conversion)]
-                    BinOp::Elvis => DynVal::from(if a.0.is_empty() { b } else { a }),
+                    BinOp::Elvis => {
+                        let is_null = matches!(serde_json::from_str(&a.0), Ok(serde_json::Value::Null));
+                        if a.0.is_empty() || is_null {
+                            b
+                        } else {
+                            a
+                        }
+                    }
                     BinOp::RegexMatch => {
                         let regex = regex::Regex::new(&b.as_string()?)?;
                         DynVal::from(regex.is_match(&a.as_string()?))
@@ -218,9 +226,12 @@ impl SimplExpr {
                     no.eval(values)
                 }
             }
-            SimplExpr::JsonAccess(span, val, index) => {
+            SimplExpr::JsonAccess(span, safe, val, index) => {
                 let val = val.eval(values)?;
                 let index = index.eval(values)?;
+
+                let is_safe = *safe == AccessType::Safe;
+
                 match val.as_json_value()? {
                     serde_json::Value::Array(val) => {
                         let index = index.as_i32()?;
@@ -234,6 +245,10 @@ impl SimplExpr {
                             .unwrap_or(&serde_json::Value::Null);
                         Ok(DynVal::from(indexed_value).at(*span))
                     }
+                    serde_json::Value::String(val) if val.is_empty() && is_safe => {
+                        Ok(DynVal::from(&serde_json::Value::Null).at(*span))
+                    }
+                    serde_json::Value::Null if is_safe => Ok(DynVal::from(&serde_json::Value::Null).at(*span)),
                     _ => Err(EvalError::CannotIndex(format!("{}", val)).at(*span)),
                 }
             }
@@ -330,3 +345,55 @@ fn call_expr_function(name: &str, args: Vec<DynVal>) -> Result<DynVal, EvalError
         _ => Err(EvalError::UnknownFunction(name.to_string())),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::dynval::DynVal;
+
+    macro_rules! evals_as {
+        ($name:ident($simplexpr:expr) => $expected:expr $(,)?) => {
+            #[test]
+            fn $name() {
+                let expected: Result<$crate::dynval::DynVal, $crate::eval::EvalError> = $expected;
+
+                let parsed = match $crate::parser::parse_string(0, 0, $simplexpr.into()) {
+                    Ok(it) => it,
+                    Err(e) => {
+                        panic!("Could not parse input as SimpleExpr\nInput: {}\nReason: {}", stringify!($simplexpr), e);
+                    }
+                };
+
+                eprintln!("Parsed as {parsed:#?}");
+
+                let output = parsed.eval_no_vars();
+
+                match expected {
+                    Ok(expected) => {
+                        let actual = output.expect("Output was not Ok(_)");
+
+                        assert_eq!(expected, actual);
+                    }
+                    Err(expected) => {
+                        let actual = output.expect_err("Output was not Err(_)").to_string();
+                        let expected = expected.to_string();
+
+                        assert_eq!(expected, actual);
+                    }
+                }
+            }
+        };
+
+        ($name:ident($simplexpr:expr) => $expected:expr, $($tt:tt)+) => {
+            evals_as!($name($simplexpr) => $expected);
+            evals_as!($($tt)*);
+        }
+    }
+
+    evals_as! {
+        string_to_string(r#""Hello""#) => Ok(DynVal::from("Hello".to_string())),
+        safe_access_to_existing(r#"{ "a": { "b": 2 } }.a?.b"#) => Ok(DynVal::from(2)),
+        safe_access_to_missing(r#"{ "a": { "b": 2 } }.b?.b"#) => Ok(DynVal::from(&serde_json::Value::Null)),
+        normal_access_to_existing(r#"{ "a": { "b": 2 } }.a.b"#) => Ok(DynVal::from(2)),
+        normal_access_to_missing(r#"{ "a": { "b": 2 } }.b.b"#) => Err(super::EvalError::CannotIndex("null".to_string())),
+    }
+}

crates/simplexpr/src/parser/lexer.rs

@@ -28,6 +28,7 @@ pub enum Token {
     GT,
     LT,
     Elvis,
+    SafeAccess,
     RegexMatch,
 
     Not,
@@ -88,6 +89,7 @@ regex_rules! {
     r">"     => |_| Token::GT,
     r"<"     => |_| Token::LT,
     r"\?:"    => |_| Token::Elvis,
+    r"\?\."    => |_| Token::SafeAccess,
     r"=~"    => |_| Token::RegexMatch,
 
     r"!"     => |_| Token::Not,
@@ -318,5 +320,6 @@ mod test {
             "${ {"hi": "ho"}.hi }".hi
         "#),
         empty_interpolation   => v!(r#""${}""#),
+        safe_interpolation   => v!(r#""${ { "key": "value" }.key1?.key2 ?: "Recovery" }""#),
     }
 }

crates/simplexpr/src/parser/mod.rs

@@ -41,6 +41,7 @@ mod tests {
             "foo.bar[2 + 2] * asdf[foo.bar]",
             r#"[1, 2, 3 + 4, "bla", [blub, blo]]"#,
             r#"{ "key": "value", 5: 1+2, true: false }"#,
+            r#"{ "key": "value" }?.key?.does_not_exist"#,
         );
     }
 }

crates/simplexpr/src/parser/snapshots/simplexpr__parser__lexer__test__safe_interpolation.snap

@@ -0,0 +1,5 @@
+---
+source: crates/simplexpr/src/parser/lexer.rs
+expression: "v!(r#\"\"${ { \"key\": \"value\" }.key1?.key2 ?: \"Recovery\" }\"\"#)"
+---
+(0, StringLit([(0, Literal(""), 3), (3, Interp([(4, LCurl, 5), (6, StringLit([(6, Literal("key"), 11)]), 11), (11, Colon, 12), (13, StringLit([(13, Literal("value"), 20)]), 20), (21, RCurl, 22), (22, Dot, 23), (23, Ident("key1"), 27), (27, SafeAccess, 29), (29, Ident("key2"), 33), (34, Elvis, 36), (37, StringLit([(37, Literal("Recovery"), 47)]), 47)]), 48), (48, Literal(""), 50)]), 50)

crates/simplexpr/src/parser/snapshots/simplexpr__parser__tests__test-16.snap

@@ -0,0 +1,7 @@
+---
+source: crates/simplexpr/src/parser/mod.rs
+expression: "p.parse(0, Lexer::new(0, 0, r#\"{ \"key\": \"value\" }?.key?.does_not_exist\"#))"
+---
+Ok(
+    {"key": "value"}?.["key"]?.["does_not_exist"],
+)

crates/simplexpr/src/simplexpr_parser.lalrpop

@@ -1,4 +1,4 @@
-use crate::ast::{SimplExpr::{self, *}, BinOp::*, UnaryOp::*};
+use crate::ast::{SimplExpr::{self, *}, BinOp::*, UnaryOp::*, AccessType};
 use eww_shared_util::{Span, VarName};
 use crate::parser::lexer::{Token, LexicalError, StrLitSegment, Sp};
 use crate::parser::lalrpop_helpers::*;
@@ -25,6 +25,7 @@ extern {
     ">"  => Token::GT,
     "<"  => Token::LT,
     "?:" => Token::Elvis,
+    "?." => Token::SafeAccess,
     "=~" => Token::RegexMatch,
 
     "!"  => Token::Not,
@@ -75,10 +76,16 @@ pub Expr: SimplExpr = {
 
   #[precedence(level="1")] #[assoc(side="right")]
   <l:@L> <ident:"identifier"> "(" <args: Comma<ExprReset>> ")" <r:@R> => FunctionCall(Span(l, r, fid), ident, args),
-  <l:@L> <value:Expr>         "[" <index: ExprReset>       "]" <r:@R> => JsonAccess(Span(l, r, fid), b(value), b(index)),
+  <l:@L> <value:Expr>         "[" <index: ExprReset>       "]" <r:@R> => {
+    JsonAccess(Span(l, r, fid), AccessType::Normal, b(value), b(index))
+  },
 
   <l:@L> <value:Expr> "." <lit_l:@L> <index:"identifier"> <r:@R> => {
-    JsonAccess(Span(l, r, fid), b(value), b(Literal(index.into())))
+    JsonAccess(Span(l, r, fid), AccessType::Normal, b(value), b(Literal(index.into())))
+  },
+
+  <l:@L> <value:Expr> "?." <lit_l:@L> <index:"identifier"> <r:@R> => {
+    JsonAccess(Span(l, r, fid), AccessType::Safe, b(value), b(Literal(index.into())))
   },
 
   #[precedence(level="2")] #[assoc(side="right")]

docs/src/expression_language.md

@@ -24,7 +24,14 @@ Supported currently are the following features:
 - comparisons (`==`, `!=`, `>`, `<`, `<=`, `>=`)
 - boolean operations (`||`, `&&`, `!`)
 - elvis operator (`?:`)
-    - if the left side is `""`, then returns the right side, otherwise evaluates to the left side.
+    - if the left side is `""` or a JSON `null`, then returns the right side,
+      otherwise evaluates to the left side.
+- Safe Access operator (`?.`)
+    - if the left side is `""` or a JSON `null`, then return `null`. Otherwise,
+      attempt to index.
+    - This can still cause an error to occur if the left hand side exists but is
+      not an object.
+      (`Number` or `String`).
 - conditionals (`condition ? 'value' : 'other value'`)
 - numbers, strings, booleans and variable references (`12`, `'hi'`, `true`, `some_variable`)
 - json access (`object.field`, `array[12]`, `object["field"]`)