Add prosemirror package for parsing and rendering Tiptap JSON to HTML

Introduces pkg/prosemirror with Go structs to unmarshal ProseMirror/Tiptap
JSON documents and an HTML renderer. Supports all standard node types
(headings, paragraphs, lists, tables, code blocks, images, blockquotes)
and mark types (bold, italic, underline, strike, code, link).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: codenem

pkg/prosemirror/html.go

@@ -0,0 +1,268 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package prosemirror
+
+import (
+	"bytes"
+	"fmt"
+	"html"
+	"strconv"
+)
+
+// RenderHTML renders a ProseMirror document node tree to an HTML string.
+func RenderHTML(node Node) (string, error) {
+	var buf bytes.Buffer
+	if err := renderNode(&buf, node); err != nil {
+		return "", err
+	}
+	return buf.String(), nil
+}
+
+func renderNode(buf *bytes.Buffer, n Node) error {
+	switch n.Type {
+	case NodeDoc:
+		return renderChildren(buf, n.Content)
+	case NodeParagraph:
+		buf.WriteString("<p>")
+		if err := renderChildren(buf, n.Content); err != nil {
+			return err
+		}
+		buf.WriteString("</p>")
+	case NodeHeading:
+		attrs, err := n.HeadingAttrs()
+		if err != nil {
+			return fmt.Errorf("cannot render heading node: %w", err)
+		}
+		if attrs.Level < 1 || attrs.Level > 6 {
+			return fmt.Errorf("cannot render heading node: invalid level %d", attrs.Level)
+		}
+		level := strconv.Itoa(attrs.Level)
+		buf.WriteString("<h")
+		buf.WriteString(level)
+		buf.WriteByte('>')
+		if err := renderChildren(buf, n.Content); err != nil {
+			return err
+		}
+		buf.WriteString("</h")
+		buf.WriteString(level)
+		buf.WriteByte('>')
+	case NodeBlockquote:
+		buf.WriteString("<blockquote>")
+		if err := renderChildren(buf, n.Content); err != nil {
+			return err
+		}
+		buf.WriteString("</blockquote>")
+	case NodeCodeBlock:
+		attrs, err := n.CodeBlockAttrs()
+		if err != nil {
+			return fmt.Errorf("cannot render code block node: %w", err)
+		}
+		buf.WriteString("<pre><code")
+		if attrs.Language != nil {
+			writeAttr(buf, "class", "language-"+*attrs.Language)
+		}
+		buf.WriteByte('>')
+		if err := renderChildren(buf, n.Content); err != nil {
+			return err
+		}
+		buf.WriteString("</code></pre>")
+	case NodeHorizontalRule:
+		buf.WriteString("<hr>")
+	case NodeHardBreak:
+		buf.WriteString("<br>")
+	case NodeText:
+		return renderText(buf, n)
+	case NodeImage:
+		attrs, err := n.ImageAttrs()
+		if err != nil {
+			return fmt.Errorf("cannot render image node: %w", err)
+		}
+		buf.WriteString("<img")
+		writeAttr(buf, "src", attrs.Src)
+		if attrs.Alt != nil {
+			writeAttr(buf, "alt", *attrs.Alt)
+		}
+		if attrs.Title != nil {
+			writeAttr(buf, "title", *attrs.Title)
+		}
+		buf.WriteByte('>')
+	case NodeBulletList:
+		buf.WriteString("<ul>")
+		if err := renderChildren(buf, n.Content); err != nil {
+			return err
+		}
+		buf.WriteString("</ul>")
+	case NodeOrderedList:
+		attrs, err := n.OrderedListAttrs()
+		if err != nil {
+			return fmt.Errorf("cannot render ordered list node: %w", err)
+		}
+		buf.WriteString("<ol")
+		if attrs.Start != 1 {
+			writeAttr(buf, "start", strconv.Itoa(attrs.Start))
+		}
+		if attrs.Type != nil {
+			writeAttr(buf, "type", *attrs.Type)
+		}
+		buf.WriteByte('>')
+		if err := renderChildren(buf, n.Content); err != nil {
+			return err
+		}
+		buf.WriteString("</ol>")
+	case NodeListItem:
+		buf.WriteString("<li>")
+		if err := renderChildren(buf, n.Content); err != nil {
+			return err
+		}
+		buf.WriteString("</li>")
+	case NodeTable:
+		buf.WriteString("<table>")
+		if err := renderChildren(buf, n.Content); err != nil {
+			return err
+		}
+		buf.WriteString("</table>")
+	case NodeTableRow:
+		buf.WriteString("<tr>")
+		if err := renderChildren(buf, n.Content); err != nil {
+			return err
+		}
+		buf.WriteString("</tr>")
+	case NodeTableCell:
+		return renderTableCell(buf, n, "td")
+	case NodeTableHeader:
+		return renderTableCell(buf, n, "th")
+	default:
+		return fmt.Errorf("cannot render node: unknown type %q", n.Type)
+	}
+	return nil
+}
+
+func renderChildren(buf *bytes.Buffer, nodes []Node) error {
+	for _, child := range nodes {
+		if err := renderNode(buf, child); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func renderText(buf *bytes.Buffer, n Node) error {
+	if n.Text == nil {
+		return fmt.Errorf("cannot render text node: text is nil")
+	}
+	for _, m := range n.Marks {
+		if err := openMark(buf, m); err != nil {
+			return err
+		}
+	}
+	buf.WriteString(html.EscapeString(*n.Text))
+	for i := len(n.Marks) - 1; i >= 0; i-- {
+		closeMark(buf, n.Marks[i])
+	}
+	return nil
+}
+
+func openMark(buf *bytes.Buffer, m Mark) error {
+	switch m.Type {
+	case MarkStrong:
+		buf.WriteString("<strong>")
+	case MarkEm:
+		buf.WriteString("<em>")
+	case MarkUnderline:
+		buf.WriteString("<u>")
+	case MarkStrike:
+		buf.WriteString("<s>")
+	case MarkCode:
+		buf.WriteString("<code>")
+	case MarkLink:
+		attrs, err := m.LinkAttrs()
+		if err != nil {
+			return fmt.Errorf("cannot render link mark: %w", err)
+		}
+		buf.WriteString("<a")
+		writeAttr(buf, "href", attrs.Href)
+		if attrs.Target != nil {
+			writeAttr(buf, "target", *attrs.Target)
+		}
+		if attrs.Rel != nil {
+			writeAttr(buf, "rel", *attrs.Rel)
+		}
+		if attrs.Class != nil {
+			writeAttr(buf, "class", *attrs.Class)
+		}
+		if attrs.Title != nil {
+			writeAttr(buf, "title", *attrs.Title)
+		}
+		buf.WriteByte('>')
+	default:
+		return fmt.Errorf("cannot render mark: unknown type %q", m.Type)
+	}
+	return nil
+}
+
+func closeMark(buf *bytes.Buffer, m Mark) {
+	switch m.Type {
+	case MarkStrong:
+		buf.WriteString("</strong>")
+	case MarkEm:
+		buf.WriteString("</em>")
+	case MarkUnderline:
+		buf.WriteString("</u>")
+	case MarkStrike:
+		buf.WriteString("</s>")
+	case MarkCode:
+		buf.WriteString("</code>")
+	case MarkLink:
+		buf.WriteString("</a>")
+	}
+}
+
+func renderTableCell(buf *bytes.Buffer, n Node, tag string) error {
+	attrs, err := n.TableCellAttrs()
+	if err != nil {
+		return fmt.Errorf("cannot render %s node: %w", tag, err)
+	}
+	buf.WriteByte('<')
+	buf.WriteString(tag)
+	if attrs.Colspan > 1 {
+		writeAttr(buf, "colspan", strconv.Itoa(attrs.Colspan))
+	}
+	if attrs.Rowspan > 1 {
+		writeAttr(buf, "rowspan", strconv.Itoa(attrs.Rowspan))
+	}
+	if len(attrs.Colwidth) > 0 {
+		total := 0
+		for _, w := range attrs.Colwidth {
+			total += w
+		}
+		writeAttr(buf, "style", "min-width: "+strconv.Itoa(total)+"px")
+	}
+	buf.WriteByte('>')
+	if err := renderChildren(buf, n.Content); err != nil {
+		return err
+	}
+	buf.WriteString("</")
+	buf.WriteString(tag)
+	buf.WriteByte('>')
+	return nil
+}
+
+func writeAttr(buf *bytes.Buffer, name, value string) {
+	buf.WriteByte(' ')
+	buf.WriteString(name)
+	buf.WriteString(`="`)
+	buf.WriteString(html.EscapeString(value))
+	buf.WriteByte('"')
+}