diff --git a/src/main.ts b/src/main.ts
index 111bc29..626c31b 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,5 +1,4 @@
 export { parseDocument } from './parser';
 export type { ParseDocumentResult } from './parser';
 export type { FoundSyntax, ParseError } from './parse_errors';
-export { ConcreteSyntax, Expr } from './syntax';
-export type { ConcreteSyntax as ConcreteSyntaxNode, Expr as ExprNode } from './syntax';
+export * from './syntax';
diff --git a/src/parse_errors.ts b/src/parse_errors.ts
index 5599ffb..d53f775 100644
--- a/src/parse_errors.ts
+++ b/src/parse_errors.ts
@@ -7,14 +7,21 @@ export type ParseError =
     found: FoundSyntax;
   }
 | {
-    tag: "expected-close-paren";
+    tag: "expected-close-delimiter";
     span: CodePointSpan;
-    openParen: CodePointSpan;
+    open: CodePointSpan;
+    expected: "paren" | "bracket";
     found: FoundSyntax;
   }
 | {
-    tag: "unexpected-close-paren";
+    tag: "unexpected-close-delimiter";
     span: CodePointSpan;
+    delimiter: "paren" | "bracket";
+  }
+| {
+    tag: "expected-list-separator";
+    span: CodePointSpan;
+    found: FoundSyntax;
   }
 | {
     tag: "unexpected-code-point";
@@ -25,7 +32,7 @@ export type ParseError =
     tag: "invalid-number";
     span: CodePointSpan;
     text: string;
-    reason: "unsafe-integer";
+    reason: "unsafe-integer" | "identifier-suffix";
   };
 
 export type FoundSyntax =
diff --git a/src/parser.experiments.ts b/src/parser.experiments.ts
index 9c5469f..6d3922e 100644
--- a/src/parser.experiments.ts
+++ b/src/parser.experiments.ts
@@ -1,7 +1,7 @@
 import { CodePointString, sourceText } from 'source-region';
 import { parseDocument } from './parser';
 import { matchCodePointString } from './recognizers';
-import { Expr } from './syntax';
+import { Program, programOf } from './syntax';
 
 // === Experiments ===
 
@@ -10,7 +10,7 @@ function experiment00_emptyDocument(): void {
 }
 
 function experiment01_topLevelExpressions(): void {
-  logParse("top-level expressions", "foo 123 (bar baz_1 qux-2)");
+  logParse("top-level expressions", "foo 123 (bar baz_1 qux-2) [a, b, c]");
 }
 
 function experiment02_nestedLists(): void {
@@ -42,12 +42,24 @@ function experiment07_matchCodePointString(): void {
   console.log("cursor", cursor.current());
 }
 
+function experiment08_squareListSeparator(): void {
+  logParse("square list separator", "[a, b c, d]");
+}
+
+function experiment09_invalidNumberFragment(): void {
+  logParse("invalid number fragment", "123fasd");
+}
+
+function experiment10_repeatedLeadingComma(): void {
+  logParse("repeated leading comma", "[, , foo, bar]");
+}
+
 function logParse(name: string, input: string): void {
   const region = sourceText(input).fullRegion();
   const result = parseDocument(region);
   console.log(`==== parser:${name} ====`);
   console.log(input);
-  console.log(result.values.map(Expr.show));
+  console.log(result.syntax.tag, Program.show(programOf(result.syntax)));
   console.dir(result.errors, { depth: null });
 }
 
@@ -60,4 +72,7 @@ function logParse(name: string, input: string): void {
   experiment05_recoverInsideList,
   experiment06_unicodeSpans,
   experiment07_matchCodePointString,
+  experiment08_squareListSeparator,
+  experiment09_invalidNumberFragment,
+  experiment10_repeatedLeadingComma,
 ].forEach((experiment) => experiment());
diff --git a/src/parser.ts b/src/parser.ts
index da158f9..bb0ada3 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -14,20 +14,34 @@ import type {
 } from 'source-region';
 import type { FoundSyntax, ParseError } from './parse_errors';
 import { consumeWhile, consumeWhile1, skipWhile } from './recognizers';
-import { ConcreteSyntax } from './syntax';
+import {
+  ConcreteError,
+  ConcreteSyntaxResult,
+  DelimiterToken,
+  Expr,
+  ListItem,
+  Program,
+} from './syntax';
+import type {
+  ConcreteInfo,
+  ListItem as ListItemType,
+  PartialConcreteSyntax,
+  ValidConcreteSyntax,
+  Expr as ExprType,
+} from './syntax';
 
 // Whitespace convention:
 // - parseDocument consumes leading whitespace before each top-level expression.
 // - parseExpr assumes leading whitespace has already been consumed.
 // - Successful expression parsers stop immediately after the expression.
-// - parseList owns whitespace between list elements and before the closing paren.
+// - list parsers own whitespace between list items and before the closing delimiter.
 //
 // Recovery policy:
-// - At document level, invalid input is skipped until EOF or a plausible expression
-//   start. Unexpected ")" is reported and consumed immediately.
-// - Inside lists, invalid input is skipped until EOF, ")", or a plausible
-//   expression start. Recovery always consumes at least one code point when it
-//   cannot stop at a synchronization point.
+// - Unknown expressions consume at least one code point, then panic until a
+//   delimiter, whitespace, or plausible expression start.
+// - Round lists do not require separators.
+// - Square lists require commas between neighboring expressions, but allow
+//   optional leading and trailing commas.
 //
 // Span convention:
 // - Parser internals and diagnostics use CodePointSpan.
@@ -35,14 +49,20 @@ import { ConcreteSyntax } from './syntax';
 
 const OPEN_PAREN = char('(');
 const CLOSE_PAREN = char(')');
+const OPEN_BRACKET = char('[');
+const CLOSE_BRACKET = char(']');
+const COMMA = char(',');
 const DASH = char('-');
 const UNDERSCORE = char('_');
 
 export type ParseDocumentResult = {
-  values: ConcreteSyntax[];
+  syntax: ConcreteSyntaxResult;
   errors: ParseError[];
 };
 
+type PartialExpr = ExprType<ConcreteInfo, ConcreteError>;
+type PartialListItem = ListItemType<ConcreteInfo, ConcreteError>;
+
 export function parseDocument(region: SourceRegion): ParseDocumentResult {
   return new Parser(region).parseDocument();
 }
@@ -56,152 +76,260 @@ class Parser {
   }
 
   parseDocument(): ParseDocumentResult {
-    const values: ConcreteSyntax[] = [];
+    const expressions: PartialExpr[] = [];
 
     while (true) {
       this.skipWhitespace();
       if (this.cursor.isAtEnd()) break;
 
-      const before = this.cursor.checkpoint();
-      const value = this.parseExpr();
-      if (value) {
-        values.push(value);
-        continue;
-      }
-
-      this.recoverDocument(before);
+      expressions.push(this.parseExpr());
     }
 
-    return { values, errors: this.errors };
+    const program = Program.make(expressions, { span: this.region.codePointSpan });
+    return {
+      syntax: this.errors.length === 0
+        ? ConcreteSyntaxResult.valid(program as ValidConcreteSyntax)
+        : ConcreteSyntaxResult.invalid(program as PartialConcreteSyntax),
+      errors: this.errors,
+    };
   }
 
-  private parseExpr(): ConcreteSyntax | undefined {
+  private parseExpr(): PartialExpr {
     const cp = this.cursor.peek();
 
     if (cp === undefined) {
-      this.errors.push({
+      return this.errorExpression(this.makeError({
         tag: "expected-expression",
         span: this.cursor.eofSpan(),
         found: this.found(),
-      });
-      return undefined;
+      }));
     }
 
-    if (cp === CLOSE_PAREN) {
-      this.errors.push({
-        tag: "unexpected-close-paren",
-        span: this.cursor.currentSpan(),
-      });
-      return undefined;
+    if (cp === CLOSE_PAREN || cp === CLOSE_BRACKET) {
+      const delimiter = cp === CLOSE_PAREN ? "paren" : "bracket";
+      const span = this.cursor.currentSpan();
+      this.cursor.advance();
+      return this.errorExpression(this.makeError({
+        tag: "unexpected-close-delimiter",
+        span,
+        delimiter,
+      }));
     }
 
-    if (cp === OPEN_PAREN) return this.parseList();
+    if (cp === OPEN_PAREN) return this.parseRoundList();
+    if (cp === OPEN_BRACKET) return this.parseSquareList();
     if (isDigit(cp)) return this.parseNumber();
     if (isIdentifierStart(cp)) return this.parseIdentifier();
 
-    this.errors.push({
-      tag: "expected-expression",
-      span: this.cursor.currentSpan(),
-      found: this.found(),
-    });
-    return undefined;
+    return this.parseUnknownExpression();
   }
 
-  private parseList(): ConcreteSyntax | undefined {
+  private parseRoundList(): PartialExpr {
     const start = this.cursor.checkpoint();
-    const openParen = this.cursor.currentSpan();
+    const open = DelimiterToken.openParen(this.cursor.currentSpan());
     this.cursor.advance();
 
-    const values: ConcreteSyntax[] = [];
+    const items: PartialListItem[] = [];
 
-    // === Body Parsing ===
     while (true) {
       this.skipWhitespace();
 
       const cp = this.cursor.peek();
       if (cp === CLOSE_PAREN) {
+        const close = DelimiterToken.closeParen(this.cursor.currentSpan());
         this.cursor.advance();
-        return ConcreteSyntax.list(values, this.cursor.spanFrom(start));
+        return Expr.list(open, items, this.cursor.spanFrom(start), close);
+      }
+
+      if (cp === CLOSE_BRACKET) {
+        const close = DelimiterToken.closeBracket(this.cursor.currentSpan());
+        const error = this.makeError({
+          tag: "expected-close-delimiter",
+          span: this.cursor.currentSpan(),
+          open: open.span,
+          expected: "paren",
+          found: this.found(),
+        });
+        this.cursor.advance();
+        return Expr.list(open, items, this.cursor.spanFrom(start), close, error);
       }
 
       if (cp === undefined) {
-        this.errors.push({
-          tag: "expected-close-paren",
+        const error = this.makeError({
+          tag: "expected-close-delimiter",
           span: this.cursor.eofSpan(),
-          openParen,
+          open: open.span,
+          expected: "paren",
           found: this.found(),
         });
-        return ConcreteSyntax.list(values, this.cursor.spanFrom(start));
+        return Expr.list(open, items, this.cursor.spanFrom(start), undefined, error);
       }
 
-      const before = this.cursor.checkpoint();
-      const value = this.parseExpr();
-      if (value) {
-        values.push(value);
-        continue;
-      }
-
-      this.recoverList(before);
+      items.push(this.parseExpr());
     }
   }
 
-  private parseNumber(): ConcreteSyntax {
+  private parseSquareList(): PartialExpr {
+    const start = this.cursor.checkpoint();
+    const open = DelimiterToken.openBracket(this.cursor.currentSpan());
+    this.cursor.advance();
+
+    const items: PartialListItem[] = [];
+    let sawExpression = false;
+    let sawLeadingComma = false;
+    let needsSeparator = false;
+
+    while (true) {
+      this.skipWhitespace();
+
+      const cp = this.cursor.peek();
+      if (cp === CLOSE_BRACKET) {
+        const close = DelimiterToken.closeBracket(this.cursor.currentSpan());
+        this.cursor.advance();
+        return Expr.list(open, items, this.cursor.spanFrom(start), close);
+      }
+
+      if (cp === CLOSE_PAREN) {
+        const close = DelimiterToken.closeParen(this.cursor.currentSpan());
+        const error = this.makeError({
+          tag: "expected-close-delimiter",
+          span: this.cursor.currentSpan(),
+          open: open.span,
+          expected: "bracket",
+          found: this.found(),
+        });
+        this.cursor.advance();
+        return Expr.list(open, items, this.cursor.spanFrom(start), close, error);
+      }
+
+      if (cp === undefined) {
+        const error = this.makeError({
+          tag: "expected-close-delimiter",
+          span: this.cursor.eofSpan(),
+          open: open.span,
+          expected: "bracket",
+          found: this.found(),
+        });
+        return Expr.list(open, items, this.cursor.spanFrom(start), undefined, error);
+      }
+
+      if (needsSeparator) {
+        if (cp === COMMA) {
+          this.cursor.advance();
+          needsSeparator = false;
+          continue;
+        }
+
+        const error = this.makeError({
+          tag: "expected-list-separator",
+          span: this.cursor.currentSpan(),
+          found: this.found(),
+        });
+        items.push(ListItem.errorSeparator(error, this.cursor.currentSpan()));
+        needsSeparator = false;
+        continue;
+      }
+
+      if (cp === COMMA) {
+        const commaSpan = this.cursor.currentSpan();
+        this.cursor.advance();
+
+        if (sawExpression) {
+          const error = this.makeError({
+            tag: "expected-expression",
+            span: commaSpan,
+            found: { tag: "code-point", value: COMMA, span: commaSpan },
+          });
+          items.push(this.errorExpression(error, commaSpan));
+        } else if (sawLeadingComma) {
+          const error = this.makeError({
+            tag: "expected-expression",
+            span: commaSpan,
+            found: { tag: "code-point", value: COMMA, span: commaSpan },
+          });
+          items.push(this.errorExpression(error, commaSpan));
+        } else {
+          sawLeadingComma = true;
+        }
+
+        continue;
+      }
+
+      items.push(this.parseExpr());
+      sawExpression = true;
+      needsSeparator = true;
+    }
+  }
+
+  private parseNumber(): PartialExpr {
+    const start = this.cursor.checkpoint();
     const match = consumeWhile1(this.cursor, isDigit);
     if (match.tag === "none") {
       throw new Error("parseNumber called when cursor is not at a number");
     }
 
+    if (isIdentifierStart(this.cursor.peek() ?? -1)) {
+      consumeWhile(this.cursor, isIdentifierPart);
+      const span = this.cursor.spanFrom(start);
+      const text = this.cursor.slice(span);
+      return Expr.errorNumber(this.makeError({
+        tag: "invalid-number",
+        span,
+        text,
+        reason: "identifier-suffix",
+      }), span);
+    }
+
     const { span, text } = match;
     const value = Number(text);
 
     if (!Number.isSafeInteger(value)) {
-      this.errors.push({
+      return Expr.errorNumber(this.makeError({
         tag: "invalid-number",
         span,
         text,
         reason: "unsafe-integer",
-      });
+      }), span);
     }
 
-    return ConcreteSyntax.number(value, span);
+    return Expr.number(value, span);
   }
 
-  private parseIdentifier(): ConcreteSyntax {
+  private parseIdentifier(): PartialExpr {
     const start = this.cursor.checkpoint();
     this.cursor.advance();
 
     consumeWhile(this.cursor, isIdentifierPart);
     const span = this.cursor.spanFrom(start);
-    return ConcreteSyntax.identifier(this.cursor.slice(span), span);
+    return Expr.identifier(this.cursor.slice(span), span);
   }
 
-  private recoverDocument(failedAt: CodePointIndex): void {
-    if (this.cursor.current() === failedAt) this.cursor.advance();
+  private parseUnknownExpression(): PartialExpr {
+    const start = this.cursor.checkpoint();
+    const focus = this.cursor.currentSpan();
 
-    while (!this.cursor.isAtEnd()) {
+    this.cursor.advance();
+    while (true) {
       const cp = this.cursor.peek();
-      if (cp === CLOSE_PAREN) {
-        this.errors.push({
-          tag: "unexpected-close-paren",
-          span: this.cursor.currentSpan(),
-        });
-        this.cursor.advance();
-        return;
+      if (
+        cp === undefined
+        || isAsciiWhitespace(cp)
+        || isClosingDelimiter(cp)
+        || isExpressionStart(cp)
+      ) {
+        break;
       }
-
-      if (isExpressionStart(cp)) return;
       this.cursor.advance();
     }
-  }
 
-  private recoverList(failedAt: CodePointIndex): void {
-    if (this.cursor.current() === failedAt) this.cursor.advance();
+    const panickedOver = this.cursor.spanFrom(start);
+    const error = this.makeError({
+      tag: "expected-expression",
+      span: focus,
+      found: { tag: "code-point", value: this.region.source.codePointAt(focus.start), span: focus },
+    }, panickedOver);
 
-    while (!this.cursor.isAtEnd()) {
-      const cp = this.cursor.peek();
-      if (cp === CLOSE_PAREN || isExpressionStart(cp)) return;
-      this.cursor.advance();
-    }
+    return this.errorExpression(error, panickedOver);
   }
 
   private skipWhitespace(): void {
@@ -213,10 +341,32 @@ class Parser {
     if (cp === undefined) return { tag: "eof", span: this.cursor.eofSpan() };
     return { tag: "code-point", value: cp, span: this.cursor.currentSpan() };
   }
+
+  private makeError(error: ParseError, panickedOver?: CodePointSpan): ConcreteError {
+    this.errors.push(error);
+    return ConcreteError.single({
+      span: error.span,
+      error,
+      panickedOver,
+    });
+  }
+
+  private errorExpression(error: ConcreteError, span?: CodePointSpan): PartialExpr {
+    return Expr.errorExpression(error, span ?? error[0].span);
+  }
 }
 
 function isExpressionStart(cp: CodePoint | undefined): boolean {
-  return cp !== undefined && (cp === OPEN_PAREN || isDigit(cp) || isIdentifierStart(cp));
+  return cp !== undefined && (
+    cp === OPEN_PAREN
+    || cp === OPEN_BRACKET
+    || isDigit(cp)
+    || isIdentifierStart(cp)
+  );
+}
+
+function isClosingDelimiter(cp: CodePoint): boolean {
+  return cp === CLOSE_PAREN || cp === CLOSE_BRACKET;
 }
 
 function isIdentifierStart(cp: CodePoint): boolean {
diff --git a/src/styles/syntax-pane.css b/src/styles/syntax-pane.css
index 5596238..17e7a1d 100644
--- a/src/styles/syntax-pane.css
+++ b/src/styles/syntax-pane.css
@@ -1,40 +1,36 @@
-.section-label {
-  margin: var(--gap-2) 0;
-  text-transform: uppercase;
+.structure-tree {
+  min-height: 0;
+  overflow: auto;
+  padding: var(--gap-3);
 }
 
-.error-list,
-.expr-list,
-.list-children {
-  display: flex;
-  flex-direction: column;
-  gap: var(--gap-2);
-}
-
-.error-card,
-.expr-node {
+.syntax-node {
   border: 1px solid var(--border);
   background: var(--panel-raised);
-}
-
-.error-card {
-  padding: var(--gap-3);
-  border-color: oklch(42% 0.05 28);
-  background: var(--error-bg);
-}
-
-.expr-node {
   padding: var(--gap-2);
 }
 
-.error-card:hover,
-.expr-node:hover {
+.syntax-node:hover {
   border-color: var(--border-strong);
   background: var(--accent-bg);
 }
 
+.syntax-error-node {
+  border-color: oklch(42% 0.05 28);
+  background: var(--error-bg);
+}
+
+.node-header,
+.list-node-header {
+  display: flex;
+  align-items: baseline;
+  gap: var(--gap-2);
+  min-width: 0;
+}
+
 .item-title,
-.node-kind {
+.node-kind,
+.error-title {
   color: var(--text);
   font-size: var(--text-sm);
   font-weight: 650;
@@ -42,7 +38,7 @@
 
 .node-kind {
   display: inline-flex;
-  min-width: 4.5rem;
+  min-width: 7rem;
   color: var(--accent);
 }
 
@@ -52,15 +48,65 @@
   font-size: var(--text-sm);
 }
 
-.list-node-header {
-  display: flex;
-  align-items: baseline;
-  justify-content: space-between;
-  gap: var(--gap-2);
+.item-meta {
+  color: var(--text-muted);
+  font-size: var(--text-xs);
 }
 
-.list-children {
+.list-children,
+.concrete-error-list {
+  display: flex;
+  flex-direction: column;
+  gap: var(--gap-2);
   margin-top: var(--gap-2);
   padding-left: var(--gap-3);
   border-left: 1px solid var(--border);
 }
+
+.delimiter-row,
+.span-chip-row {
+  display: flex;
+  flex-wrap: wrap;
+  gap: var(--gap-2);
+  margin-top: var(--gap-2);
+}
+
+.concrete-error {
+  padding: var(--gap-2);
+  border: 1px solid oklch(42% 0.05 28);
+  background: oklch(21% 0.035 28);
+}
+
+.span-chip {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--gap-2);
+  border: 1px solid var(--border);
+  padding: var(--gap-1) var(--gap-2);
+  color: var(--text-muted);
+  background: var(--panel);
+  font-family: var(--font-mono);
+  font-size: var(--text-xs);
+  cursor: default;
+}
+
+.span-chip:hover {
+  color: var(--text);
+  border-color: var(--accent);
+  background: var(--accent-bg);
+}
+
+.status-dot {
+  width: 0.65rem;
+  height: 0.65rem;
+  flex: 0 0 auto;
+  border-radius: 50%;
+}
+
+.status-valid {
+  background: oklch(72% 0.12 150);
+}
+
+.status-invalid {
+  background: var(--error);
+}
diff --git a/src/syntax.ts b/src/syntax.ts
index bdbf4ad..bbf5f22 100644
--- a/src/syntax.ts
+++ b/src/syntax.ts
@@ -1,59 +1,171 @@
 import type { CodePointSpan } from 'source-region';
+import type { ParseError } from './parse_errors';
 
-export type ConcreteSyntax = Expr<{ span: CodePointSpan }>
+export type ConcreteInfo = { span: CodePointSpan };
 
-export type Expr<A> =
-| { tag: "literal", value: Literal } & A
-| { tag: "list", values: Expr<A>[] } & A
+export type ConcreteSyntaxResult =
+| { tag: "valid", value: ValidConcreteSyntax }
+| { tag: "invalid", value: PartialConcreteSyntax }
 
-export namespace ConcreteSyntax {
-  export function number(value: number, span: CodePointSpan): ConcreteSyntax {
-    return { tag: "literal", value: { tag: "number", value }, span };
+export type ValidConcreteSyntax = Program<ConcreteInfo, never>
+export type PartialConcreteSyntax = Program<ConcreteInfo, ConcreteError>
+
+export type ConcreteError = ConcreteErrorNode[] // Convention: can't be empty.
+export type ConcreteErrorNode = {
+  span: CodePointSpan,
+  error: ParseError,
+  panickedOver?: CodePointSpan,
+}
+
+export namespace ConcreteError {
+  export function single(node: ConcreteErrorNode): ConcreteError {
+    return [node];
+  }
+}
+
+export type DelimiterToken =
+  | { tag: "open-paren"; span: CodePointSpan }
+  | { tag: "close-paren"; span: CodePointSpan }
+  | { tag: "open-bracket"; span: CodePointSpan }
+  | { tag: "close-bracket"; span: CodePointSpan };
+
+export namespace DelimiterToken {
+  export function openParen(span: CodePointSpan): DelimiterToken {
+    return { tag: "open-paren", span };
   }
 
-  export function identifier(value: Identifier, span: CodePointSpan): ConcreteSyntax {
-    return { tag: "literal", value: { tag: "identifier", value }, span };
+  export function closeParen(span: CodePointSpan): DelimiterToken {
+    return { tag: "close-paren", span };
   }
 
-  export function list(values: ConcreteSyntax[], span: CodePointSpan): ConcreteSyntax {
-    return { tag: "list", values, span };
+  export function openBracket(span: CodePointSpan): DelimiterToken {
+    return { tag: "open-bracket", span };
+  }
+
+  export function closeBracket(span: CodePointSpan): DelimiterToken {
+    return { tag: "close-bracket", span };
+  }
+}
+
+export type Program<Info, Error> = {
+  tag: "program",
+  expressions: Expr<Info, Error>[],
+  error?: Error,
+} & Info
+
+export type Expr<Info, Error> =
+| Literal<Info, Error>
+| List<Info, Error>
+| { tag: "error-expression", error: Error } & Info
+
+export type List<Info, Error> =
+  { tag: "list", open: DelimiterToken, items: ListItem<Info, Error>[], close?: DelimiterToken, error?: Error } & Info
+
+export type ListItem<Info, Error> =
+| Expr<Info, Error>
+| { tag: "error-list-separator", error: Error } & Info
+
+export type Literal<Info, Error> =
+| { tag: "number", value: number } & Info
+| { tag: "error-number", error: Error } & Info
+| { tag: "identifier", value: Identifier } & Info
+| { tag: "error-identifier", error: Error } & Info
+
+export type Identifier = string
+
+export namespace ConcreteSyntaxResult {
+  export function valid(value: ValidConcreteSyntax): ConcreteSyntaxResult {
+    return { tag: "valid", value };
+  }
+
+  export function invalid(value: PartialConcreteSyntax): ConcreteSyntaxResult {
+    return { tag: "invalid", value };
+  }
+}
+
+export namespace Program {
+  export function make<Info, Error>(
+    expressions: Expr<Info, Error>[],
+    info: Info,
+    error?: Error,
+  ): Program<Info, Error> {
+    return error === undefined
+      ? { tag: "program", expressions, ...info }
+      : { tag: "program", expressions, error, ...info };
+  }
+
+  export function show<Info, Error>(program: Program<Info, Error>): string {
+    return program.expressions.map(Expr.show).join(" ");
   }
 }
 
 export namespace Expr {
-  export function number(value: number): Expr<void> {
-    return { tag: "literal", value: { tag: "number", value } };
+  export function number(value: number, span: CodePointSpan): Expr<ConcreteInfo, ConcreteError> {
+    return { tag: "number", value, span };
   }
 
-  export function identifier(value: Identifier): Expr<void> {
-    return { tag: "literal", value: { tag: "identifier", value } };
+  export function errorNumber(error: ConcreteError, span: CodePointSpan): Expr<ConcreteInfo, ConcreteError> {
+    return { tag: "error-number", error, span };
   }
 
-  export function list(values: Expr<void>[]): Expr<void> {
-    return { tag: "list", values };
+  export function identifier(value: Identifier, span: CodePointSpan): Expr<ConcreteInfo, ConcreteError> {
+    return { tag: "identifier", value, span };
   }
 
-  export function show<A>(e: Expr<A>): string {
-    switch (e.tag) {
-      case "literal":
-        return showLiteral(e.value);
-      case "list":
-        return `(${e.values.map(show).join(" ")})`;
-    }
+  export function errorIdentifier(error: ConcreteError, span: CodePointSpan): Expr<ConcreteInfo, ConcreteError> {
+    return { tag: "error-identifier", error, span };
   }
 
-  function showLiteral(e: Literal): string {
-    switch (e.tag) {
+  export function list(
+    open: DelimiterToken,
+    items: ListItem<ConcreteInfo, ConcreteError>[],
+    span: CodePointSpan,
+    close?: DelimiterToken,
+    error?: ConcreteError,
+  ): Expr<ConcreteInfo, ConcreteError> {
+    return { tag: "list", open, items, close, error, span };
+  }
+
+  export function errorExpression(error: ConcreteError, span: CodePointSpan): Expr<ConcreteInfo, ConcreteError> {
+    return { tag: "error-expression", error, span };
+  }
+
+  export function show<Info, Error>(expr: Expr<Info, Error>): string {
+    switch (expr.tag) {
       case "number":
-        return `${e.value}`;
+        return `${expr.value}`;
       case "identifier":
-        return `${e.value}`;
+        return expr.value;
+      case "error-number":
+        return "<error-number>";
+      case "error-identifier":
+        return "<error-identifier>";
+      case "error-expression":
+        return "<error-expression>";
+      case "list":
+        return showList(expr);
     }
   }
+
+  function showList<Info, Error>(list: List<Info, Error>): string {
+    const open = list.open.tag === "open-bracket" ? "[" : "(";
+    const close = list.open.tag === "open-bracket" ? "]" : ")";
+    const sep = list.open.tag === "open-bracket" ? ", " : " ";
+    return `${open}${list.items.map(ListItem.show).join(sep)}${close}`;
+  }
 }
 
-type Literal =
-| { tag: "number", value: number }
-| { tag: "identifier", value: Identifier }
+export namespace ListItem {
+  export function errorSeparator(error: ConcreteError, span: CodePointSpan): ListItem<ConcreteInfo, ConcreteError> {
+    return { tag: "error-list-separator", error, span };
+  }
 
-type Identifier = string
+  export function show<Info, Error>(item: ListItem<Info, Error>): string {
+    if (item.tag === "error-list-separator") return "<error-separator>";
+    return Expr.show(item);
+  }
+}
+
+export function programOf(result: ConcreteSyntaxResult): PartialConcreteSyntax {
+  return result.value;
+}
diff --git a/src/ui/App.tsx b/src/ui/App.tsx
index 5c8103e..5483dc1 100644
--- a/src/ui/App.tsx
+++ b/src/ui/App.tsx
@@ -1,31 +1,35 @@
-import { createMemo, createSignal, Show } from 'solid-js';
+import { createMemo, createSignal } from 'solid-js';
 import { sourceText } from 'source-region';
 import type { CodePointSpan, SourceRegion, SourceText } from 'source-region';
 import { parseDocument } from '../parser';
 import type { ParseError } from '../parse_errors';
-import type { ConcreteSyntax } from '../syntax';
+import { programOf } from '../syntax';
+import type { ConcreteSyntaxResult, PartialConcreteSyntax } from '../syntax';
 import { spanLabel } from './format';
 import { PaneHeader, PaneSplitter } from './Pane';
 import { SourceGrid } from './SourceGrid';
 import type { SourceGridAnnotation } from './SourceGrid';
-import { ErrorList, ExpressionList } from './SyntaxPane';
+import { StructureTree } from './SyntaxPane';
 import type { HoverTarget } from './types';
 
 type ParsedDocument = {
   source: SourceText;
   region: SourceRegion;
-  values: ConcreteSyntax[];
+  syntax: ConcreteSyntaxResult;
+  program: PartialConcreteSyntax;
   errors: ParseError[];
 };
 
 const SAMPLE_INPUT = `(define square (_ x) (mul x x))
 
-(add 1 2)
+[add, 1, 2]
 
 (define pyth (_ x y) (+ (square x) (square y)))
 
 foo ) @@@ (bar 1)
-(nested (list 123 abc_9 name-with-dash))`;
+(nested [list, 123, abc_9, name-with-dash])
+[a, b c, d]
+123fasd`;
 
 export function App() {
   const [input, setInput] = createSignal(SAMPLE_INPUT);
@@ -37,7 +41,7 @@ export function App() {
     const source = sourceText(input());
     const region = source.fullRegion();
     const result = parseDocument(region);
-    return { source, region, values: result.values, errors: result.errors };
+    return { source, region, syntax: result.syntax, program: programOf(result.syntax), errors: result.errors };
   });
 
   return (
@@ -71,23 +75,14 @@ export function App() {
       <section class="pane structure-pane">
         <PaneHeader
           title="Structure"
-          detail={`${parsed().values.length} expressions, ${parsed().errors.length} errors`}
+          detail={`${parsed().syntax.tag}, ${parsed().program.expressions.length} expressions, ${parsed().errors.length} errors`}
+        />
+        <StructureTree
+          program={parsed().program}
+          isValid={parsed().syntax.tag === "valid"}
+          errorCount={parsed().errors.length}
+          onHover={setHovered}
         />
-        <Show
-          when={parsed().errors.length > 0}
-          fallback={
-            <ExpressionList
-              values={parsed().values}
-              onHover={setHovered}
-            />
-          }
-        >
-          <ErrorList
-            errors={parsed().errors}
-            values={parsed().values}
-            onHover={setHovered}
-          />
-        </Show>
       </section>
 
       <PaneSplitter
diff --git a/src/ui/SyntaxPane.tsx b/src/ui/SyntaxPane.tsx
index 2e7e325..79faeab 100644
--- a/src/ui/SyntaxPane.tsx
+++ b/src/ui/SyntaxPane.tsx
@@ -1,55 +1,195 @@
 import { For, Show } from 'solid-js';
 import type { JSX } from 'solid-js';
 import type { CodePointSpan } from 'source-region';
-import type { ParseError } from '../parse_errors';
-import type { ConcreteSyntax } from '../syntax';
+import type {
+  ConcreteError,
+  ConcreteErrorNode,
+  ConcreteInfo,
+  List,
+  ListItem,
+  PartialConcreteSyntax,
+  Expr as SyntaxExpr,
+} from '../syntax';
 import { Expr } from '../syntax';
-import { errorDetail, errorLabel, errorTitle } from './format';
+import { errorDetail, errorTitle, spanLabel } from './format';
 import type { HoverTarget } from './types';
 
-export function ErrorList(props: {
-  errors: ParseError[];
-  values: ConcreteSyntax[];
+type PartialExpr = SyntaxExpr<ConcreteInfo, ConcreteError>;
+type PartialList = List<ConcreteInfo, ConcreteError>;
+type PartialListItem = ListItem<ConcreteInfo, ConcreteError>;
+
+export function StructureTree(props: {
+  program: PartialConcreteSyntax;
+  isValid: boolean;
+  errorCount: number;
   onHover: (target: HoverTarget | undefined) => void;
 }) {
   return (
-    <div class="scroll-stack">
-      <div class="section-label">Errors</div>
-      <div class="error-list">
-        <For each={props.errors}>
-          {(error) => (
-            <HoverBlock
-              class="error-card"
-              label={errorLabel(error)}
-              span={error.span}
-              onHover={props.onHover}
-            >
-              <div class="item-title">{errorTitle(error)}</div>
-              <div class="item-meta">{errorDetail(error)}</div>
-            </HoverBlock>
-          )}
-        </For>
-      </div>
+    <div class="structure-tree">
+      <HoverBlock
+        class="syntax-node program-node"
+        label="program"
+        span={props.program.span}
+        onHover={props.onHover}
+      >
+        <div class="node-header">
+          <span class={props.isValid ? "status-dot status-valid" : "status-dot status-invalid"} />
+          <span class="node-kind">program</span>
+          <span class="item-meta">
+            {props.isValid ? "valid" : "invalid"} · {props.program.expressions.length} expressions · {props.errorCount} errors
+          </span>
+        </div>
 
-      <Show when={props.values.length > 0}>
-        <div class="section-label">Recovered Expressions</div>
-        <ExpressionList values={props.values} onHover={props.onHover} />
-      </Show>
+        <Show when={props.program.error}>
+          {(error) => <ConcreteErrorView error={error()} label="program error" onHover={props.onHover} />}
+        </Show>
+
+        <div class="list-children">
+          <For each={props.program.expressions}>
+            {(expr, index) => (
+              <ExprView
+                expr={expr}
+                label={`expression ${index() + 1}`}
+                onHover={props.onHover}
+              />
+            )}
+          </For>
+        </div>
+      </HoverBlock>
     </div>
   );
 }
 
-export function ExpressionList(props: {
-  values: ConcreteSyntax[];
+function ExprView(props: {
+  expr: PartialExpr;
+  label: string;
+  onHover: (target: HoverTarget | undefined) => void;
+}) {
+  switch (props.expr.tag) {
+    case "number":
+    case "identifier":
+      return (
+        <HoverBlock
+          class="syntax-node literal-node"
+          label={`${props.label}: ${props.expr.tag}`}
+          span={props.expr.span}
+          onHover={props.onHover}
+        >
+          <div class="node-header">
+            <span class="node-kind">{props.expr.tag}</span>
+            <span class="node-value">{Expr.show(props.expr)}</span>
+          </div>
+        </HoverBlock>
+      );
+
+    case "error-number":
+    case "error-identifier":
+    case "error-expression":
+      return (
+        <HoverBlock
+          class="syntax-node syntax-error-node"
+          label={`${props.label}: ${props.expr.tag}`}
+          span={props.expr.span}
+          onHover={props.onHover}
+        >
+          <div class="node-header">
+            <span class="status-dot status-invalid" />
+            <span class="node-kind">{props.expr.tag}</span>
+            <span class="item-meta">{spanLabel(props.expr.span)}</span>
+          </div>
+          <ConcreteErrorView error={props.expr.error} label={props.expr.tag} onHover={props.onHover} />
+        </HoverBlock>
+      );
+
+    case "list":
+      return <ListView list={props.expr} label={props.label} onHover={props.onHover} />;
+  }
+}
+
+function ListView(props: {
+  list: PartialList;
+  label: string;
   onHover: (target: HoverTarget | undefined) => void;
 }) {
   return (
-    <div class="expr-list">
-      <For each={props.values}>
-        {(value, index) => (
-          <ExprView
-            expr={value}
-            label={`expression ${index() + 1}`}
+    <HoverBlock
+      class={props.list.error ? "syntax-node list-node syntax-error-node" : "syntax-node list-node"}
+      label={`${props.label}: list`}
+      span={props.list.span}
+      onHover={props.onHover}
+    >
+      <div class="node-header">
+        <Show when={props.list.error}>
+          <span class="status-dot status-invalid" />
+        </Show>
+        <span class="node-kind">{listLabel(props.list.open.tag)}</span>
+        <span class="item-meta">{props.list.items.length} items · {delimiterLabel(props.list)}</span>
+      </div>
+
+      <div class="delimiter-row">
+        <SpanChip label={props.list.open.tag} span={props.list.open.span} onHover={props.onHover} />
+        <Show when={props.list.close}>
+          {(close) => <SpanChip label={close().tag} span={close().span} onHover={props.onHover} />}
+        </Show>
+      </div>
+
+      <Show when={props.list.error}>
+        {(error) => <ConcreteErrorView error={error()} label="list error" onHover={props.onHover} />}
+      </Show>
+
+      <div class="list-children">
+        <For each={props.list.items}>
+          {(item, index) => (
+            <ListItemView
+              item={item}
+              label={`${props.label}.${index() + 1}`}
+              onHover={props.onHover}
+            />
+          )}
+        </For>
+      </div>
+    </HoverBlock>
+  );
+}
+
+function ListItemView(props: {
+  item: PartialListItem;
+  label: string;
+  onHover: (target: HoverTarget | undefined) => void;
+}) {
+  if (props.item.tag === "error-list-separator") {
+    return (
+      <HoverBlock
+        class="syntax-node syntax-error-node"
+        label={`${props.label}: separator`}
+        span={props.item.span}
+        onHover={props.onHover}
+      >
+        <div class="node-header">
+          <span class="status-dot status-invalid" />
+          <span class="node-kind">error-list-separator</span>
+          <span class="item-meta">{spanLabel(props.item.span)}</span>
+        </div>
+        <ConcreteErrorView error={props.item.error} label="separator error" onHover={props.onHover} />
+      </HoverBlock>
+    );
+  }
+
+  return <ExprView expr={props.item} label={props.label} onHover={props.onHover} />;
+}
+
+function ConcreteErrorView(props: {
+  error: ConcreteError;
+  label: string;
+  onHover: (target: HoverTarget | undefined) => void;
+}) {
+  return (
+    <div class="concrete-error-list">
+      <For each={props.error}>
+        {(node, index) => (
+          <ConcreteErrorNodeView
+            node={node}
+            label={`${props.label} ${index() + 1}`}
             onHover={props.onHover}
           />
         )}
@@ -58,48 +198,40 @@ export function ExpressionList(props: {
   );
 }
 
-function ExprView(props: {
-  expr: ConcreteSyntax;
+function ConcreteErrorNodeView(props: {
+  node: ConcreteErrorNode;
   label: string;
   onHover: (target: HoverTarget | undefined) => void;
 }) {
-  if (props.expr.tag === "literal") {
-    return (
-      <HoverBlock
-        class="expr-node literal-node"
-        label={`${props.label}: ${props.expr.value.tag}`}
-        span={props.expr.span}
-        onHover={props.onHover}
-      >
-        <span class="node-kind">{props.expr.value.tag}</span>
-        <span class="node-value">{literalValue(props.expr)}</span>
-      </HoverBlock>
-    );
-  }
-
   return (
-    <HoverBlock
-      class="expr-node list-node"
-      label={`${props.label}: list`}
-      span={props.expr.span}
-      onHover={props.onHover}
+    <div class="concrete-error">
+      <div class="error-title">{errorTitle(props.node.error)}</div>
+      <div class="item-meta">{errorDetail(props.node.error)}</div>
+      <div class="span-chip-row">
+        <SpanChip label="focus" span={props.node.span} onHover={props.onHover} />
+        <Show when={props.node.panickedOver}>
+          {(span) => <SpanChip label="panicked over" span={span()} onHover={props.onHover} />}
+        </Show>
+      </div>
+    </div>
+  );
+}
+
+function SpanChip(props: {
+  label: string;
+  span: CodePointSpan;
+  onHover: (target: HoverTarget | undefined) => void;
+}) {
+  return (
+    <button
+      class="span-chip"
+      type="button"
+      onMouseEnter={() => props.onHover({ label: props.label, span: props.span })}
+      onMouseLeave={() => props.onHover(undefined)}
     >
-      <div class="list-node-header">
-        <span class="node-kind">list</span>
-        <span class="item-meta">{props.expr.values.length} children</span>
-      </div>
-      <div class="list-children">
-        <For each={props.expr.values}>
-          {(child, index) => (
-            <ExprView
-              expr={child}
-              label={`${props.label}.${index() + 1}`}
-              onHover={props.onHover}
-            />
-          )}
-        </For>
-      </div>
-    </HoverBlock>
+      <span>{props.label}</span>
+      <span>{spanLabel(props.span)}</span>
+    </button>
   );
 }
 
@@ -121,6 +253,10 @@ function HoverBlock(props: {
   );
 }
 
-function literalValue(expr: ConcreteSyntax): string {
-  return expr.tag === "literal" ? Expr.show(expr) : "";
+function listLabel(tag: string): string {
+  return tag === "open-bracket" ? "square-list" : "round-list";
+}
+
+function delimiterLabel(list: PartialList): string {
+  return list.close ? `${list.open.tag} / ${list.close.tag}` : `${list.open.tag} / missing close`;
 }
diff --git a/src/ui/format.ts b/src/ui/format.ts
index 63d8cf2..8722db6 100644
--- a/src/ui/format.ts
+++ b/src/ui/format.ts
@@ -5,10 +5,12 @@ export function errorTitle(error: ParseError): string {
   switch (error.tag) {
     case "expected-expression":
       return "Expected expression";
-    case "expected-close-paren":
-      return "Expected closing paren";
-    case "unexpected-close-paren":
-      return "Unexpected closing paren";
+    case "expected-close-delimiter":
+      return "Expected closing delimiter";
+    case "unexpected-close-delimiter":
+      return "Unexpected closing delimiter";
+    case "expected-list-separator":
+      return "Expected list separator";
     case "unexpected-code-point":
       return "Unexpected code point";
     case "invalid-number":
@@ -24,10 +26,12 @@ export function errorDetail(error: ParseError): string {
   switch (error.tag) {
     case "expected-expression":
       return `found ${foundLabel(error.found)}`;
-    case "expected-close-paren":
-      return `opened at ${spanLabel(error.openParen)}, found ${foundLabel(error.found)}`;
-    case "unexpected-close-paren":
-      return spanLabel(error.span);
+    case "expected-close-delimiter":
+      return `expected ${error.expected}, opened at ${spanLabel(error.open)}, found ${foundLabel(error.found)}`;
+    case "unexpected-close-delimiter":
+      return `${error.delimiter} ${spanLabel(error.span)}`;
+    case "expected-list-separator":
+      return `found ${foundLabel(error.found)}`;
     case "unexpected-code-point":
       return `found ${foundLabel(error.found)}`;
     case "invalid-number":