Implement default values for at() (#995)

7 files changed, 76 insertions, 27 deletions

diff --git a/src/eval/array.rs b/src/eval/array.rs
index f6e2f2d4..6ae5d7cf 100644
--- a/src/eval/array.rs
+++ b/src/eval/array.rs
@@ -74,10 +74,15 @@ impl Array {
     }
 
     /// Borrow the value at the given index.
-    pub fn at(&self, index: i64) -> StrResult<&Value> {
+    pub fn at<'a>(
+        &'a self,
+        index: i64,
+        default: Option<&'a Value>,
+    ) -> StrResult<&'a Value> {
         self.locate(index)
             .and_then(|i| self.0.get(i))
-            .ok_or_else(|| out_of_bounds(index, self.len()))
+            .or(default)
+            .ok_or_else(|| out_of_bounds_no_default(index, self.len()))
     }
 
     /// Mutably borrow the value at the given index.
@@ -85,7 +90,7 @@ impl Array {
         let len = self.len();
         self.locate(index)
             .and_then(move |i| self.0.make_mut().get_mut(i))
-            .ok_or_else(|| out_of_bounds(index, len))
+            .ok_or_else(|| out_of_bounds_no_default(index, len))
     }
 
     /// Push a value to the end of the array.
@@ -462,3 +467,14 @@ fn array_is_empty() -> EcoString {
 fn out_of_bounds(index: i64, len: i64) -> EcoString {
     eco_format!("array index out of bounds (index: {}, len: {})", index, len)
 }
+
+/// The out of bounds access error message when no default value was given.
+#[cold]
+fn out_of_bounds_no_default(index: i64, len: i64) -> EcoString {
+    eco_format!(
+        "array index out of bounds (index: {}, len: {}) \
+         and no default value was specified",
+        index,
+        len
+    )
+}
diff --git a/src/eval/dict.rs b/src/eval/dict.rs
index b137f03c..1b28a6ba 100644
--- a/src/eval/dict.rs
+++ b/src/eval/dict.rs
@@ -53,16 +53,20 @@ impl Dict {
         self.0.len() as i64
     }
 
-    /// Borrow the value the given `key` maps to.
-    pub fn at(&self, key: &str) -> StrResult<&Value> {
-        self.0.get(key).ok_or_else(|| missing_key(key))
+    /// Borrow the value the given `key` maps to,
+    pub fn at<'a>(
+        &'a self,
+        key: &str,
+        default: Option<&'a Value>,
+    ) -> StrResult<&'a Value> {
+        self.0.get(key).or(default).ok_or_else(|| missing_key_no_default(key))
     }
 
     /// Mutably borrow the value the given `key` maps to.
     pub fn at_mut(&mut self, key: &str) -> StrResult<&mut Value> {
         Arc::make_mut(&mut self.0)
             .get_mut(key)
-            .ok_or_else(|| missing_key(key))
+            .ok_or_else(|| missing_key_no_default(key))
     }
 
     /// Remove the value if the dictionary contains the given key.
@@ -218,3 +222,13 @@ impl<'a> IntoIterator for &'a Dict {
 fn missing_key(key: &str) -> EcoString {
     eco_format!("dictionary does not contain key {:?}", Str::from(key))
 }
+
+/// The missing key access error message when no default was fiven.
+#[cold]
+fn missing_key_no_default(key: &str) -> EcoString {
+    eco_format!(
+        "dictionary does not contain key {:?} \
+         and no default value was specified",
+        Str::from(key)
+    )
+}
diff --git a/src/eval/methods.rs b/src/eval/methods.rs
index 6cbb846b..8d042a5c 100644
--- a/src/eval/methods.rs
+++ b/src/eval/methods.rs
@@ -30,7 +30,7 @@ pub fn call(
             "len" => Value::Int(string.len()),
             "first" => Value::Str(string.first().at(span)?),
             "last" => Value::Str(string.last().at(span)?),
-            "at" => Value::Str(string.at(args.expect("index")?).at(span)?),
+            "at" => Value::Str(string.at(args.expect("index")?, None).at(span)?),
             "slice" => {
                 let start = args.expect("start")?;
                 let mut end = args.eat()?;
@@ -73,7 +73,7 @@ pub fn call(
         Value::Content(content) => match method {
             "func" => content.func().into(),
             "has" => Value::Bool(content.has(&args.expect::<EcoString>("field")?)),
-            "at" => content.at(&args.expect::<EcoString>("field")?).at(span)?,
+            "at" => content.at(&args.expect::<EcoString>("field")?, None).at(span)?,
             "location" => content
                 .location()
                 .ok_or("this method can only be called on content returned by query(..)")
@@ -86,7 +86,10 @@ pub fn call(
             "len" => Value::Int(array.len()),
             "first" => array.first().at(span)?.clone(),
             "last" => array.last().at(span)?.clone(),
-            "at" => array.at(args.expect("index")?).at(span)?.clone(),
+            "at" => array
+                .at(args.expect("index")?, args.named("default")?.as_ref())
+                .at(span)?
+                .clone(),
             "slice" => {
                 let start = args.expect("start")?;
                 let mut end = args.eat()?;
@@ -125,7 +128,10 @@ pub fn call(
 
         Value::Dict(dict) => match method {
             "len" => Value::Int(dict.len()),
-            "at" => dict.at(&args.expect::<Str>("key")?).at(span)?.clone(),
+            "at" => dict
+                .at(&args.expect::<Str>("key")?, args.named("default")?.as_ref())
+                .at(span)?
+                .clone(),
             "keys" => Value::Array(dict.keys()),
             "values" => Value::Array(dict.values()),
             "pairs" => Value::Array(dict.pairs()),
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index fe1fac3b..b430b400 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -1255,7 +1255,7 @@ impl ast::Pattern {
         for p in destruct.bindings() {
             match p {
                 ast::DestructuringKind::Normal(expr) => {
-                    let Ok(v) = value.at(i) else {
+                    let Ok(v) = value.at(i, None) else {
                         bail!(expr.span(), "not enough elements to destructure");
                     };
                     f(vm, expr, v.clone())?;
@@ -1310,17 +1310,17 @@ impl ast::Pattern {
         for p in destruct.bindings() {
             match p {
                 ast::DestructuringKind::Normal(ast::Expr::Ident(ident)) => {
-                    let Ok(v) = value.at(&ident) else {
-                                        bail!(ident.span(), "destructuring key not found in dictionary");
-                                    };
+                    let Ok(v) = value.at(&ident, None) else {
+                        bail!(ident.span(), "destructuring key not found in dictionary");
+                    };
                     f(vm, ast::Expr::Ident(ident.clone()), v.clone())?;
                     used.insert(ident.take());
                 }
                 ast::DestructuringKind::Sink(spread) => sink = spread.expr(),
                 ast::DestructuringKind::Named(named) => {
-                    let Ok(v) = value.at(named.name().as_str()) else {
-                                        bail!(named.name().span(), "destructuring key not found in dictionary");
-                                    };
+                    let Ok(v) = value.at(named.name().as_str(), None) else {
+                        bail!(named.name().span(), "destructuring key not found in dictionary");
+                    };
                     f(vm, named.expr(), v.clone())?;
                     used.insert(named.name().take());
                 }
diff --git a/src/eval/str.rs b/src/eval/str.rs
index 89be3699..d7e00bf6 100644
--- a/src/eval/str.rs
+++ b/src/eval/str.rs
@@ -69,12 +69,13 @@ impl Str {
     }
 
     /// Extract the grapheme cluster at the given index.
-    pub fn at(&self, index: i64) -> StrResult<Self> {
+    pub fn at<'a>(&'a self, index: i64, default: Option<&'a str>) -> StrResult<Self> {
         let len = self.len();
         let grapheme = self.0[self.locate(index)?..]
             .graphemes(true)
             .next()
-            .ok_or_else(|| out_of_bounds(index, len))?;
+            .or(default)
+            .ok_or_else(|| no_default_and_out_of_bounds(index, len))?;
         Ok(grapheme.into())
     }
 
@@ -348,6 +349,12 @@ fn out_of_bounds(index: i64, len: i64) -> EcoString {
     eco_format!("string index out of bounds (index: {}, len: {})", index, len)
 }
 
+/// The out of bounds access error message when no default value was given.
+#[cold]
+fn no_default_and_out_of_bounds(index: i64, len: i64) -> EcoString {
+    eco_format!("no default value was specified and string index out of bounds (index: {}, len: {})", index, len)
+}
+
 /// The char boundary access error message.
 #[cold]
 fn not_a_char_boundary(index: i64) -> EcoString {
diff --git a/src/eval/value.rs b/src/eval/value.rs
index 0548c01f..1bfad9c8 100644
--- a/src/eval/value.rs
+++ b/src/eval/value.rs
@@ -124,8 +124,8 @@ impl Value {
     pub fn field(&self, field: &str) -> StrResult<Value> {
         match self {
             Self::Symbol(symbol) => symbol.clone().modified(field).map(Self::Symbol),
-            Self::Dict(dict) => dict.at(field).cloned(),
-            Self::Content(content) => content.at(field),
+            Self::Dict(dict) => dict.at(field, None).cloned(),
+            Self::Content(content) => content.at(field, None),
             Self::Module(module) => module.get(field).cloned(),
             v => Err(eco_format!("cannot access fields on type {}", v.type_name())),
         }
diff --git a/src/model/content.rs b/src/model/content.rs
index 4af4e655..1bd19f14 100644
--- a/src/model/content.rs
+++ b/src/model/content.rs
@@ -252,8 +252,10 @@ impl Content {
     }
 
     /// Borrow the value of the given field.
-    pub fn at(&self, field: &str) -> StrResult<Value> {
-        self.field(field).ok_or_else(|| missing_field(field))
+    pub fn at(&self, field: &str, default: Option<Value>) -> StrResult<Value> {
+        self.field(field)
+            .or(default)
+            .ok_or_else(|| missing_field_no_default(field))
     }
 
     /// The content's label.
@@ -582,8 +584,12 @@ impl Fold for Vec<Meta> {
     }
 }
 
-/// The missing key access error message.
+/// The missing key access error message when no default value was given.
 #[cold]
-fn missing_field(key: &str) -> EcoString {
-    eco_format!("content does not contain field {:?}", Str::from(key))
+fn missing_field_no_default(key: &str) -> EcoString {
+    eco_format!(
+        "content does not contain field {:?} and \
+         no default value was specified",
+        Str::from(key)
+    )
 }