error handling

sample/handle.pl.xml

@@ -0,0 +1,26 @@
+<program name="handle">
+    <main>
+        <handle>
+            <try>
+                <integer value="non" />
+                <call function="print-line">
+                    <arguments>
+                        <string value="there is no bug" />
+                    </arguments>
+                </call>
+            </try>
+            <catch variable="err">
+                <call function="print-line">
+                    <arguments>
+                        <string value="oh no there is a bug" />
+                    </arguments>
+                </call>
+                <call function="print-line">
+                    <arguments>
+                        <value variable="err" />
+                    </arguments>
+                </call>
+            </catch>
+        </handle>
+    </main>
+</program>

src/error.rs

@@ -17,7 +17,7 @@ pub struct MissingAttribute(pub &'static str, pub &'static str);
 
 impl fmt::Display for MissingAttribute {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "missing '{}' child in '{}' node", self.1, self.0)
+        write!(f, "missing '{}' attribute in '{}' node", self.1, self.0)
     }
 }
 
@@ -118,3 +118,14 @@ impl fmt::Display for UnknownVariable {
 }
 
 impl Error for UnknownVariable {}
+
+#[derive(Clone, Debug)]
+pub struct InaccessibleFile(pub String);
+
+impl fmt::Display for InaccessibleFile {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "unable to access file '{}'", self.0)
+    }
+}
+
+impl Error for InaccessibleFile {}

src/instruction.rs

@@ -43,6 +43,7 @@ pub enum Instruction {
     },
     Each(String, Box<Instruction>, Vec<Instruction>),
     While(Box<Instruction>, Vec<Instruction>),
+    Handle(Vec<Instruction>, Vec<Instruction>, String),
 }
 
 impl Instruction {
@@ -230,6 +231,19 @@ impl Instruction {
                     util::find_node(&node, "do").ok_or(MissingChild("while", "from"))?,
                 )?,
             ),
+            "handle" => Instruction::Handle(
+                Instruction::from_children(
+                    util::find_node(&node, "try").ok_or(MissingChild("handle", "try"))?,
+                )?,
+                Instruction::from_children(
+                    util::find_node(&node, "catch").ok_or(MissingChild("handle", "try"))?,
+                )?,
+                util::find_node(&node, "catch")
+                    .ok_or(MissingChild("handle", "try"))?
+                    .attribute("variable")
+                    .ok_or(MissingAttribute("catch", "variable"))?
+                    .to_string(),
+            ),
             tag => Err(format!("unknown tag '{}'", tag))?,
         })
     }
@@ -698,6 +712,18 @@ impl Instruction {
                     }
                     None
                 }
+                Instruction::Handle(try_block, catch_block, variable) => {
+                    for ins in try_block {
+                        if let Err(e) = ins.run(ctx, globals) {
+                            ctx.assign(variable.clone(), Value::String(e.to_string()));
+                            for ins in catch_block {
+                                ins.run(ctx, globals)?;
+                            }
+                            break;
+                        }
+                    }
+                    None
+                }
             }
         } else {
             None

src/stl.rs

@@ -1,3 +1,5 @@
+use crate::error::InaccessibleFile;
+
 use super::error::{BadArgumentCount, InvalidArgument};
 use super::{Context, Value};
 use std::cell::RefCell;
@@ -232,12 +234,12 @@ fn write_file(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
                     .open(path)
                 {
                     if let Ok(_) = write!(file, "{}", contents) {
-                        Ok(Some(Value::Integer(1)))
+                        Ok(None)
                     } else {
-                        Ok(Some(Value::Integer(0)))
+                        Err(InaccessibleFile(path.clone()).into())
                     }
                 } else {
-                    Ok(Some(Value::Integer(0)))
+                    Err(InaccessibleFile(path.clone()).into())
                 }
             } else {
                 Err(InvalidArgument("write-file", "string").into())
@@ -256,7 +258,7 @@ fn read_file(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             if let Ok(contents) = fs::read_to_string(path) {
                 Ok(Some(Value::String(contents)))
             } else {
-                Ok(None)
+                Err(InaccessibleFile(path.clone()).into())
             }
         } else {
             Err(InvalidArgument("read-file", "string").into())