Use plz notation in error messages

src/language/semantics/key-path.ts

@@ -1,4 +1,7 @@
+import { either } from '../../adts.js'
 import type { Atom, Molecule } from '../parsing.js'
+import { unparse } from '../unparsing.js'
+import { prettyPlz } from '../unparsing/pretty-plz.js'
 
 export const functionParameter = Symbol('functionParameter')
 export const functionReturn = Symbol('functionReturn')
@@ -15,7 +18,14 @@ export type KeyPath = readonly (
 )[]
 
 export const stringifyKeyPathForEndUser = (keyPath: KeyPath): string =>
-  JSON.stringify(keyPath)
+  either.match(
+    // TODO: Use single-line plz notation.
+    unparse(keyPathToMolecule(keyPath), prettyPlz),
+    {
+      right: stringifiedOutput => stringifiedOutput,
+      left: error => `(unserializable key path: ${error.message})`,
+    },
+  )
 
 export const keyPathToMolecule = (keyPath: KeyPath): Molecule =>
   Object.fromEntries(

src/language/semantics/semantic-graph.ts

@@ -6,6 +6,8 @@ import type {
 } from '../errors.js'
 import type { Atom, Molecule } from '../parsing.js'
 import type { Canonicalized } from '../parsing/syntax-tree.js'
+import { unparse } from '../unparsing.js'
+import { prettyPlz } from '../unparsing/pretty-plz.js'
 import { serializeFunctionNode, type FunctionNode } from './function-node.js'
 import { stringifyKeyPathForEndUser, type KeyPath } from './key-path.js'
 import {
@@ -174,7 +176,17 @@ export const serialize = (
 
 export const stringifySemanticGraphForEndUser = (
   graph: SemanticGraph,
-): string => JSON.stringify(serialize(graph))
+): string =>
+  either.match(
+    either.flatMap(serialize(graph), output =>
+      // TODO: Use single-line plz notation.
+      unparse(output, prettyPlz),
+    ),
+    {
+      right: stringifiedOutput => stringifiedOutput,
+      left: error => `(unserializable value: ${error.message})`,
+    },
+  )
 
 export const isSemanticGraph = (
   value: