minor fixes

cozowebui/src/App.js

@@ -69,10 +69,10 @@ function App() {
     </Cell>
 
     function displayValue(v) {
-        if (v instanceof Object) {
-            return JSON.stringify(v)
-        } else {
+        if (typeof v === 'string') {
             return v
+        } else {
+            return <span style={{color: "#184A90"}}>{JSON.stringify(v)}</span>
         }
     }
 

src/parse/query.rs

@@ -6,12 +6,12 @@ use std::fmt::{Display, Formatter};
 
 use either::Left;
 use itertools::Itertools;
-use miette::{bail, ensure, Diagnostic, LabeledSpan, Report, Result};
+use miette::{bail, Diagnostic, ensure, LabeledSpan, Report, Result};
 use smartstring::{LazyCompact, SmartString};
 use thiserror::Error;
 
 use crate::algo::{AlgoHandle, AlgoNotFoundError};
-use crate::data::aggr::{parse_aggr, Aggregation};
+use crate::data::aggr::{Aggregation, parse_aggr};
 use crate::data::expr::Expr;
 use crate::data::id::Validity;
 use crate::data::program::{
@@ -22,9 +22,9 @@ use crate::data::program::{
 use crate::data::symb::Symbol;
 use crate::data::tuple::Tuple;
 use crate::data::value::DataValue;
+use crate::parse::{ExtractSpan, Pair, Pairs, ParseError, Rule, SourceSpan};
 use crate::parse::expr::build_expr;
 use crate::parse::pull::parse_out_options;
-use crate::parse::{ExtractSpan, Pair, Pairs, ParseError, Rule, SourceSpan};
 use crate::runtime::relation::{RelationId, RelationMetadata};
 
 #[derive(Error, Diagnostic, Debug)]
@@ -423,7 +423,16 @@ fn parse_rule(
     let span = src.extract_span();
     let mut src = src.into_inner();
     let head = src.next().unwrap();
+    let head_span = head.extract_span();
     let (name, head, aggr) = parse_rule_head(head, param_pool)?;
+
+    #[derive(Debug, Error, Diagnostic)]
+    #[error("Horn-clause rule cannot have empty rule head")]
+    #[diagnostic(code(parser::empty_horn_rule_head))]
+    struct EmptyRuleHead(#[label] SourceSpan);
+
+    ensure!(!head.is_empty(), EmptyRuleHead(head_span));
+
     let mut at = None;
     let mut body = src.next().unwrap();
     if body.as_rule() == Rule::expr {

src/query/compile.rs

@@ -441,8 +441,11 @@ impl SessionTx {
 
         let cur_ret_set: BTreeSet<_> = ret.bindings_after_eliminate().into_iter().collect();
         #[derive(Debug, Error, Diagnostic)]
-        #[error("Symbol {0} in rule head is unbound")]
+        #[error("Symbol '{0}' in rule head is unbound")]
         #[diagnostic(code(eval::unbound_symb_in_head))]
+        #[diagnostic(help(
+            "Note that symbols occurring only in negated positions are not considered bound"
+        ))]
         struct UnboundSymbolInRuleHead(String, #[label] SourceSpan);
 
         ensure!(cur_ret_set == ret_vars_set, {

src/query/reorder.rs

@@ -8,11 +8,12 @@ use crate::data::program::{NormalFormAtom, NormalFormRule};
 use crate::parse::SourceSpan;
 
 #[derive(Diagnostic, Debug, Error)]
-#[error("Encountered unsafe negation")]
+#[error("Encountered unsafe negation, or empty rule definition")]
 #[diagnostic(code(eval::unsafe_negation))]
 #[diagnostic(help(
-"Only rule applications that are partially bounded, \
-                or expressions / unifications that are completely bounded, can be safely negated."
+    "Only rule applications that are partially bounded, \
+or expressions / unifications that are completely bounded, can be safely negated. \
+You may also encounter this error if your rule can never produce any rows."
 ))]
 pub(crate) struct UnsafeNegation(#[label] pub(crate) SourceSpan);