There was no version of the markdown filter available for Drupal 7, and no existing issue on this in the issue queue at Drupal.org. I wasn't in an urgent need for this module, but I will need it on a project that is hoping to go D7 as soon as it is released, so I thought I'd use it as an excuse look further into the API changes in Drupal 7.

I'm working on the alpha2 release of Drupal7 and the D6 version of markdown checked out from CVS. I've posted the code up on drupal.org. Here's what needed to be done:

Update: It's now been accepted and commited to Drupal CVS!

Markdown is a simple module, with just a couple of files that needed editing. First up is the module info file. This needs updating to core 7.x and the new files[] option must be set, because Drupal 7 doesn't include any module files by default in attempts to cut down on request overhead. Here's exactly what needs adding to the markdown.info file:

Now to look at the main module file. This file needs a complete overhaul to work with Drupal 7 because hook_filter() and hook_filter_tips() has been replaced by hook_filter_info(). The markdown module also provides a block with extra help information, and this code needs changing to work with D7 because hook_block() has been replaced by a family of functions: hook_block_info(), hook_block_configure(), hook_block_save(), and hook_block_view().

Design pattern changes between Drupal 6 and Drupal 7 APIs

Comparing the changes in hook_filter() and hook_block() with their new D7 counterparts reveals interesting changes in the Drupal API patterns. A common pattern in the Drupal 6 APIs is the use of an $op parameter when calling a hook. In the implementation of the hook this would usually be utilised in a switch statement from which your code would know what it was expected to do or what it was return.

In the new hook_filter_info() the pattern used is to return the names of a series of callback functions that will be used to perform various actions required of filters.

An alternative pattern can be seen in the way that hook_block() has been replaced. In this case the branches of the original hook have been replaced by a family of hooks.

What does this mean for Markdown?

The code from the functions that were called from the original hook_filter() can be reused as callbacks as specified in the array returned from hook_filter_info(). That pretty much completes the migration of the filter bit of this module.

The code from hook_block() is split out into separate functions for markdown_block_info() where the block is defined, and markdown_block_view() where the block is rendered.

Another gottcha:

There's one other small tweak that was needed, in our beloved FAPI (Forms API) there has been a small change in how the array keys for 'item' and 'markup' items are defined. In this case the markdown module uses an 'item' form-item, so we have to change the '#value' array key to '#markup'.

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I've submitted this for review over in the Markdown filter issue queue on Drupal.org and hopefully it will eventually surface as a Drupal 7 module.