Return error if there is a String data type definition

src/lib.rs

@@ -39,7 +39,7 @@ pub fn compile_book(book: &mut Book, opt_level: OptimizationLevel) -> Result<Com
 pub fn desugar_book(book: &mut Book, opt_level: OptimizationLevel) -> Result<DefId, String> {
   let main = book.check_has_main()?;
   book.check_shared_names()?;
-  book.encode_strs();
+  book.encode_strs()?;
   book.generate_scott_adts();
   book.resolve_refs();
   encode_pattern_matching(book)?;

src/term/transform/encode_strs.rs

@@ -10,7 +10,11 @@ const HEAD: &str = "head";
 const TAIL: &str = "tail";
 
 impl Book {
-  pub fn encode_strs(&mut self) {
+  pub fn encode_strs(&mut self) -> Result<(), String> {
+    if self.adts.contains_key(&Name::new(STRING)) {
+      return Err(format!("String is a built-in data type and should not be overridden."));
+    }
+
     self.ctrs.insert(Name::new(SNIL), Name::new(STRING));
     self.ctrs.insert(Name::new(SCONS), Name::new(STRING));
 
@@ -33,6 +37,8 @@ impl Book {
       self.ctrs.remove(&Name::new(SCONS));
       self.adts.remove(&Name::new(STRING));
     }
+
+    Ok(())
   }
 }
 
@@ -64,7 +70,7 @@ impl Term {
         let fst_uses = fst.encode_str();
         let snd_uses = snd.encode_str();
         fst_uses || snd_uses
-      },
+      }
       Term::Match { arms, .. } => {
         let mut used = false;
         for arm in arms {

tests/golden_tests.rs

@@ -160,7 +160,7 @@ fn encode_pattern_match() {
   run_golden_test_dir(function_name!(), &|code| {
     let mut book = do_parse_book(code)?;
     book.check_shared_names()?;
-    book.encode_strs();
+    book.encode_strs()?;
     book.generate_scott_adts();
     book.resolve_refs();
     encode_pattern_matching(&mut book)?;

tests/golden_tests/compile_file/adt_string.hvm

@@ -0,0 +1,3 @@
+data String = S
+
+main = S

tests/snapshots/compile_file__adt_string.hvm.snap

@@ -0,0 +1,5 @@
+---
+source: tests/golden_tests.rs
+input_file: tests/golden_tests/compile_file/adt_string.hvm
+---
+String is a built-in data type and should not be overridden.