termaid

Render Mermaid diagrams as Unicode box-drawing art, suitable for a terminal.

termaid parses a subset of Mermaid — flowcharts, state diagrams, class diagrams, entity-relationship diagrams, and sequence diagrams — and lays them out as box-drawing art. Output is plain UTF-8 lines plus a TUI-agnostic, class-tagged span model for callers that want to colorize. It depends on no terminal framework; unsupported diagram types fall back to a framed raw box.

Example

Pipe a Mermaid diagram into termaid and it prints box-drawing art — here it is rendering its own pipeline:

$ termaid <<'EOF'
flowchart TD
  Src[Mermaid text] --> R{render}
  R -->|recognized| P[parse]
  R -->|no match| F[fallback]
  P --> L[layout]
  L --> Cv[canvas]
  Cv --> Art[box art]
  F --> Art
EOF
   ┌──────────────┐
   │ Mermaid text │
   └───────┬──────┘
           │
           ▼
      ╭────────╮
      │ render │
      ╰────┬───╯
     ┌─────┴──────┐
     ▼recognized  ▼no match
 ┌───────┐  ┌──────────┐
 │ parse │  │ fallback ├─────┐
 └───┬───┘  └──────────┘     │
     └─────┐                 │
           ▼                 │
      ┌────────┐             │
      │ layout │             │
      └────┬───┘             │
           │                 │
           ▼                 │
      ┌────────┐             │
      │ canvas │             │
      └────┬───┘             │
           │                 │
           ▼                 │
      ┌─────────┐            │
      │ box art │◄───────────┘
      └─────────┘

API

type cls = Empty | Border | Text | Edge | Edge_label | Title
type span = { text : string; cls : cls }
type line = span list
type t = { plain_lines : string list; styled_lines : line list }

val render : ?max_width:int -> string -> t option

render returns None only for empty input; unrecognized or too-wide diagrams render as a framed fallback. Map cls to your own colors (ANSI, notty, …).

Development

A standard dune project. With a local opam switch:

$ opam install . --deps-only --with-test --with-doc   # first-time setup
$ dune build                     # library, CLI, and examples
$ dune runtest                   # invariant + golden suites
$ dune build @doc                # odoc API docs
$ dune exec examples/demo.exe    # render a few sample diagrams
$ printf 'graph TD\n A-->B\n' | dune exec bin/main.exe    # run the CLI on stdin

The golden suite in test/golden/ diffs the renderer, byte-for-byte, against a reference "oracle" — the upstream Rust renderer built under oracle/. scripts/gen_golden.sh regenerates those fixtures from the oracle, and scripts/check_upstream.sh watches upstream mermaid.rs for changes; see oracle/README.md. These dev-only tools live in the repo but are excluded from the published package tarball.

Attribution & license

termaid is licensed under the Apache License 2.0 (see LICENSE).

It is an OCaml port of the self-contained Rust mermaid.rs terminal renderer from xai-org/grok-build, which is also Apache-2.0. The original diagram parsing, layout, and box-drawing logic are the work of SpaceXAI; see NOTICE for details and a list of modifications.