Read a carry fence's type in one function

src/markdown/code-language.ts

@@ -1,5 +1,5 @@
 import type { JsonValue } from '../json-value.ts'
-import { carryFencePrefix } from './opaque-carry.ts'
+import { carryFenceType } from './opaque-carry.ts'
 import { infoStringCarries } from './commonmark/grammar.ts'
 
 export type LanguageSlot = { info: string; kind: 'fence' } | { kind: 'attribute' } | { kind: 'none' }
@@ -7,6 +7,6 @@ export type LanguageSlot = { info: string; kind: 'fence' } | { kind: 'attribute'
 // spec/flavour.md, The CommonMark blocks: the one slot a codeBlock's language rides.
 export function languageSlot(language: JsonValue | undefined): LanguageSlot {
   if (language === undefined) return { kind: 'none' }
-  if (typeof language !== 'string' || language.startsWith(carryFencePrefix) || !infoStringCarries(language)) return { kind: 'attribute' }
+  if (typeof language !== 'string' || carryFenceType(language) !== undefined || !infoStringCarries(language)) return { kind: 'attribute' }
   return { info: language, kind: 'fence' }
 }

src/markdown/opaque-carry.ts

@@ -32,6 +32,11 @@ export function carriedInline(node: AdfNode, path: ConvertErrorPath): Result<str
   return success(spellInlineLeafDirective(carryName, spellAttributes([[jsonAttribute, spellStringAttribute(json.value.json)]])))
 }
 
+// The type a fence's info string names, `undefined` where the info string opens no carry.
+export function carryFenceType(info: string): string | undefined {
+  return info.startsWith(carryFencePrefix) ? info.slice(carryFencePrefix.length) : undefined
+}
+
 export function readCarriedBlock(type: string, body: string, depth: number): Read<AdfNode> {
   if (type !== '' && !infoStringCarries(type)) {
     return { fault: unsupportedNodeShape(`the carry fence names a type no info string carries back: spell it ${carryFencePrefix} with the type in the JSON`) }

src/markdown/parse/markdown-to-adf.ts

@@ -5,7 +5,7 @@ import type { ConvertFault } from '../../result.ts'
 import type { Flavour } from '../plain-conventions.ts'
 import type { LineContainer } from '../line-container.ts'
 import type { LinkDefinitions } from './inline-content.ts'
-import { carryFencePrefix, readCarriedBlock } from '../opaque-carry.ts'
+import { carryFenceType, readCarriedBlock } from '../opaque-carry.ts'
 import { commonMarkSpelling, type SpellingMemo } from '../emit/adf-to-markdown.ts'
 import { documentAttribute, documentName, documentSpelling, listBreakName, listBreakSpelling } from '../block-directive.ts'
 import { failure, faulted, positioned, success, type ConvertErrorPath, type ParseError, type Result, type SourcePosition } from '../../result.ts'
@@ -303,8 +303,9 @@ function listNode(node: AdfNode, items: readonly Block[][], reading: Reading, pa
 }
 
 function codeBlockNode(language: string, text: string, reading: Reading, path: ConvertErrorPath, depth: number): Result<AdfNode> {
-  if (language.startsWith(carryFencePrefix)) {
+  const carriedType = carryFenceType(language)
-    const carried = readCarriedBlock(language.slice(carryFencePrefix.length), text, depth)
+  if (carriedType !== undefined) {
+    const carried = readCarriedBlock(carriedType, text, depth)
     if (carried.fault !== undefined) return faulted(carried.fault, path)
     reading.carried.add(carried.value)
     return success(carried.value)