Add a validation step back.

examples/basic/test2.ngr

@@ -0,0 +1,4 @@
+x = 5;
+x = 4*x + 3;
+print x;
+print y;

src/backend.rs

@@ -8,7 +8,7 @@ use cranelift_codegen::isa::LookupError;
 use cranelift_codegen::settings::{Configurable, SetError};
 use cranelift_codegen::{isa, settings, CodegenError};
 use cranelift_module::{default_libcall_names, ModuleCompiledFunction, ModuleError};
-use cranelift_object::{ObjectBuilder, ObjectModule, object};
+use cranelift_object::{object, ObjectBuilder, ObjectModule};
 use target_lexicon::Triple;
 use thiserror::Error;
 
@@ -36,24 +36,24 @@ pub enum BackendError {
 impl From<BackendError> for Diagnostic<usize> {
     fn from(value: BackendError) -> Self {
         match value {
-            BackendError::Cranelift(me) =>
-                Diagnostic::error()
-                    .with_message(format!("Internal cranelift error: {}", me)),
-            BackendError::BuiltinError(me) =>
-                Diagnostic::error()
-                    .with_message(format!("Internal runtime function error: {}", me)),
-            BackendError::VariableLookupFailure =>
-                Diagnostic::error()
-                    .with_message("Internal variable lookup error!"),
-            BackendError::CodegenError(me) =>
-                Diagnostic::error()
-                    .with_message(format!("Internal codegen error: {}", me)),
-            BackendError::SetError(me) =>
-                Diagnostic::error()
-                    .with_message(format!("Internal backend setup error: {}", me)),
-            BackendError::LookupError(me) =>
-                Diagnostic::error()
-                    .with_message(format!("Internal error: {}", me)),
+            BackendError::Cranelift(me) => {
+                Diagnostic::error().with_message(format!("Internal cranelift error: {}", me))
+            }
+            BackendError::BuiltinError(me) => {
+                Diagnostic::error().with_message(format!("Internal runtime function error: {}", me))
+            }
+            BackendError::VariableLookupFailure => {
+                Diagnostic::error().with_message("Internal variable lookup error!")
+            }
+            BackendError::CodegenError(me) => {
+                Diagnostic::error().with_message(format!("Internal codegen error: {}", me))
+            }
+            BackendError::SetError(me) => {
+                Diagnostic::error().with_message(format!("Internal backend setup error: {}", me))
+            }
+            BackendError::LookupError(me) => {
+                Diagnostic::error().with_message(format!("Internal error: {}", me))
+            }
         }
     }
 }

src/bin.rs

@@ -41,7 +41,9 @@ impl From<MainError> for Diagnostic<usize> {
             MainError::Backend(be) => be.into(),
             MainError::ParserError(pe) => (&pe).into(),
             MainError::IoError(e) => Diagnostic::error().with_message(format!("IO error: {}", e)),
-            MainError::WriteError(e) => Diagnostic::error().with_message(format!("Module write error: {}", e)),
+            MainError::WriteError(e) => {
+                Diagnostic::error().with_message(format!("Module write error: {}", e))
+            }
         }
     }
 }
@@ -50,13 +52,27 @@ fn compile(file_database: &mut SimpleFiles<String, String>) -> Result<(), MainEr
     let args = CommandLineArguments::parse();
 
     let syntax = Syntax::parse_file(file_database, &args.file)?;
+    let (mut errors, mut warnings) = syntax.validate();
+    let stop = !errors.is_empty();
+    let messages = errors
+        .drain(..)
+        .map(Into::into)
+        .chain(warnings.drain(..).map(Into::into));
+    let writer = StandardStream::stderr(ColorChoice::Auto);
+    let config = codespan_reporting::term::Config::default();
+
+    for message in messages {
+        term::emit(&mut writer.lock(), &config, file_database, &message).unwrap();
+    }
+
+    if stop {
+        return Ok(());
+    }
+
     let ir = IR::from(syntax.simplify());
     let compiled = Cranelift::new(Triple::host(), ir)?;
     let bytes = compiled.bytes()?;
-    std::fs::write(
-        args.output.unwrap_or_else(|| "output.o".to_string()),
-        bytes,
-    )?;
+    std::fs::write(args.output.unwrap_or_else(|| "output.o".to_string()), bytes)?;
     Ok(())
 }
 

src/syntax.rs

@@ -11,6 +11,7 @@ lalrpop_mod!(
     "/syntax/parser.rs"
 );
 pub mod ast;
+mod validate;
 
 pub use crate::syntax::ast::*;
 pub use crate::syntax::location::Location;

src/syntax/simplify.rs

@@ -1,4 +1,4 @@
-use crate::syntax::ast::{Program, Statement, Expression};
+use crate::syntax::ast::{Expression, Program, Statement};
 
 impl Program {
     pub fn simplify(mut self) -> Self {
@@ -40,7 +40,11 @@ impl Expression {
 
                 let new_name = format!("<{}:{}>", base_name, *gensym_index);
                 *gensym_index += 1;
-                prereqs.push(Statement::Binding(loc.clone(), new_name.clone(), Expression::Primitive(loc.clone(), prim, new_exprs)));
+                prereqs.push(Statement::Binding(
+                    loc.clone(),
+                    new_name.clone(),
+                    Expression::Primitive(loc.clone(), prim, new_exprs),
+                ));
 
                 (prereqs, Expression::Reference(loc, new_name))
             }

src/syntax/validate.rs

@@ -0,0 +1,100 @@
+use crate::syntax::{Expression, Location, Program, Statement};
+use codespan_reporting::diagnostic::Diagnostic;
+use std::collections::HashMap;
+
+pub enum Error {
+    UnboundVariable(Location, String),
+}
+
+impl From<Error> for Diagnostic<usize> {
+    fn from(x: Error) -> Self {
+        match &x {
+            Error::UnboundVariable(location, name) => location
+                .labelled_error("unbound here")
+                .with_message(format!("Unbound variable '{}'", name)),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum Warning {
+    ShadowedVariable(Location, Location, String),
+}
+
+impl From<Warning> for Diagnostic<usize> {
+    fn from(x: Warning) -> Self {
+        match &x {
+            Warning::ShadowedVariable(original, new, name) => Diagnostic::warning()
+                .with_labels(vec![
+                    new.primary_label().with_message("variable rebound here"),
+                    original
+                        .secondary_label()
+                        .with_message("original binding site"),
+                ])
+                .with_message(format!("Variable '{}' is rebound", name)),
+        }
+    }
+}
+
+impl Program {
+    pub fn validate(&self) -> (Vec<Error>, Vec<Warning>) {
+        let mut errors = vec![];
+        let mut warnings = vec![];
+        let mut bound_variables = HashMap::new();
+
+        for stmt in self.statements.iter() {
+            match stmt {
+                Statement::Binding(loc, var, val) => {
+                    // we're going to make the decision that a variable is not bound in the right
+                    // hand side of its binding, which makes a lot of things easier. So we'll just
+                    // immediately check the expression, and go from there.
+                    let (mut exp_errors, mut exp_warnings) = val.validate(&bound_variables);
+
+                    errors.append(&mut exp_errors);
+                    warnings.append(&mut exp_warnings);
+                    if let Some(original_binding_site) = bound_variables.get(var) {
+                        warnings.push(Warning::ShadowedVariable(
+                            original_binding_site.clone(),
+                            loc.clone(),
+                            var.clone(),
+                        ));
+                    } else {
+                        bound_variables.insert(var.clone(), loc.clone());
+                    }
+                }
+
+                Statement::Print(_, var) if bound_variables.contains_key(var) => {}
+                Statement::Print(loc, var) => {
+                    errors.push(Error::UnboundVariable(loc.clone(), var.clone()))
+                }
+            }
+        }
+
+        (errors, warnings)
+    }
+}
+
+impl Expression {
+    fn validate(&self, variable_map: &HashMap<String, Location>) -> (Vec<Error>, Vec<Warning>) {
+        match self {
+            Expression::Value(_, _) => (vec![], vec![]),
+            Expression::Reference(_, var) if variable_map.contains_key(var) => (vec![], vec![]),
+            Expression::Reference(loc, var) => (
+                vec![Error::UnboundVariable(loc.clone(), var.clone())],
+                vec![],
+            ),
+            Expression::Primitive(_, _, args) => {
+                let mut errors = vec![];
+                let mut warnings = vec![];
+
+                for expr in args.iter() {
+                    let (mut err, mut warn) = expr.validate(variable_map);
+                    errors.append(&mut err);
+                    warnings.append(&mut warn);
+                }
+
+                (errors, warnings)
+            }
+        }
+    }
+}