Reverse arrows <-- not supported in flowchart

[nourhenta]

Description

Using a reverse arrow (<--) in a Mermaid flowchart diagram leads to a cryptic syntax error. Many users may naturally assume B <-- C is valid, expecting it to behave like C --> B, especially since reverse arrows are supported in other graphing tools.

Instead of a helpful explanation, Mermaid throws the following error:

Expecting 'LINK', 'UNICODE_TEXT', 'EDGE_TEXT', got '1'

This behavior is confusing and not well-documented. Either the syntax should be supported, or Mermaid should provide a more descriptive error message.

Steps to reproduce

1. Go to https://mermaid.live/
2. Paste the following code in the editor:

flowchart TD
  A --> B
  B <-- C

3. Notice the syntax error thrown:
   Expecting 'LINK', 'UNICODE_TEXT', 'EDGE_TEXT', got '1'

Screenshots

Code Sample

Setup

• Mermaid Version: v11.5.0 (tested on Mermaid Live Editor)
• Browser: Chrome v122
• Platform: Windows 11

Suggested Solutions

• Add support for B <-- C syntax and internally convert it to C --> B.

• Improve error messaging for unsupported arrow syntax.
  Instead of a cryptic parser error, show:
  "Reverse arrows (<--) are not currently supported. Try writing C --> B instead."

Additional Context

No response

[BambioGaming]

I agree. The error message is "cryptic" because it speaks in parser language. When the parser reads B <-- C, it gets confused:

• It sees B, then expects a valid arrow direction like -->, ---, etc.
• Instead, it encounters <--, which doesn't match any valid link syntax.
• Then it sees the 1st character of the next token (likely the node name C, or number 1 in another context), and throws an error because it was expecting a LINK.

That’s why it says:
Expecting 'LINK', 'UNICODE_TEXT', 'EDGE_TEXT', got '1'

It would be much clearer if it said something like:
Unsupported arrow '<--'. Mermaid only supports arrows like '-->', '---', etc.