On the other hand, it is recommended practice to define only one data type per module, isn't it?
Only by very few people as far as I know. I know that Henning Thielemann advocates this, but it seems like a rather arbitrary restriction to me. I find it quite common to define several data types in a single module, so long as they're sufficiently related. Depending on how they are used, I may or may not export any one of them. Have a look at Basics.lhs in Haskore, then imagine separating that such that every one of those data types was in its own module. You'd end up with a lot of modules which only had one data type in them and nothing else. I think it's just better to use your own picture of how the project fits together to decide where to put things. Unless you're explicitly using modules for hiding data constructors, it doesn't seem necessary to structure your modules around your types. - Cale