On Tue, Aug 14, 2012 at 8:44 AM, Michael Snoyman
An example that came up at work (with Yitzchak Gale, he probably has more details) was essentially two different types of documents that shared a lot of the same kinds of elements (tables, lists, paragraphs, etc) but some elements only appeared in one of the document types. We needed to render to (for sake of argument) two different XML formats, and wanted to be certain we didn't put in elements from type 1 in type 2. The solution looked something like this (using data kinds and GADTs):
This is the same thing that I did on the fb library. There are two kinds of Facebook access tokens: an app access token and an user access token. Some methods require either one or the other (e.g. [5]), but there are also some methods that may use whatever kind of access token you have (e.g. [3,4]). So AccessToken [1,2] is defined as the following GADT: data AccessToken kind where UserAccessToken :: UserId -> AccessTokenData -> UTCTime -> AccessToken UserKind AppAccessToken :: AccessTokenData -> AccessToken AppKind data UserKind data AppKind (Yes, that could be a data kind!) And for convenience we also export some type synonyms: type UserAccessToken = AccessToken UserKind type AppAccessToken = AccessToken AppKind So we get the convenience of one data type and the type safety of two, which is especially nice considering that there are functions returning access tokens as well [6,7]. Cheers, =) [1] http://hackage.haskell.org/packages/archive/fb/0.9.7/doc/html/Facebook.html#... [2] http://hackage.haskell.org/packages/archive/fb/0.9.7/doc/html/src/Facebook-T... [3] http://hackage.haskell.org/packages/archive/fb/0.9.7/doc/html/Facebook.html#... [4] http://hackage.haskell.org/packages/archive/fb/0.9.7/doc/html/Facebook.html#... [5] http://hackage.haskell.org/packages/archive/fb/0.9.7/doc/html/Facebook.html#... [6] http://hackage.haskell.org/packages/archive/fb/0.9.7/doc/html/Facebook.html#... [7] http://hackage.haskell.org/packages/archive/fb/0.9.7/doc/html/Facebook.html#... -- Felipe.