error handling

src/lib.rs

@@ -1,15 +1,17 @@
 use std::error::Error;
 use std::fs;
 
-use roxmltree::{Document, Node};
+use roxmltree::Document;
 
 mod context;
+mod error;
 mod instruction;
 mod stl;
 mod util;
 mod value;
 
 use context::Context;
+use error::{InvalidProgram, MissingChild, Unnamed};
 use instruction::Instruction;
 use value::{Function, Value};
 
@@ -23,33 +25,22 @@ pub fn run(filename: &str) -> Result<(), Box<dyn Error>> {
 
     let main = root
         .first_element_child()
-        .ok_or("invalid program structure")?
+        .ok_or(InvalidProgram)?
         .children()
         .find(|node| util::tag_name(&node) == "main")
-        .ok_or("No 'main' block")?;
+        .ok_or(MissingChild("program", "main"))?;
     let main_ast = Instruction::from_children(main)?;
 
     let functions = root
         .first_element_child()
-        .ok_or("invalid program structure")?
+        .ok_or(InvalidProgram)?
         .children()
         .filter(|node| node.tag_name().name() == String::from("function"));
 
     for fun in functions {
         ctx.assign(
-            String::from(fun.attribute("name").ok_or("unnamed function")?),
-            Value::Function(Function {
-                args: util::find_node(&fun, "arguments")
-                    .ok_or("missing 'arguments' block in 'call' tag")?
-                    .children()
-                    .filter(Node::is_element)
-                    .map(|n| n.attribute("name").and_then(|s| Some(String::from(s))))
-                    .collect::<Option<Vec<String>>>()
-                    .ok_or("unnamed argument")?,
-                ins: Instruction::from_children(
-                    util::find_node(&fun, "body").ok_or("missing 'body' block in 'call' tag")?,
-                )?,
-            }),
+            String::from(fun.attribute("name").ok_or(Unnamed("function"))?),
+            Value::Function(Function::from(&fun)?),
         );
     }