On Tue, Aug 14, 2012 at 2:32 PM, Simon Peyton-Jones
<simonpj@microsoft.com> wrote:
Friends
I’m giving a series of five lectures at the
Laser Summer School (2-8 Sept), on “Adventures with types in Haskell”. My plan is:
1.
Type classes
2.
Type families [examples including Repa type tags]
3.
GADTs
4.
Kind polymorphism
5.
System FC and deferred type errors
This message is to invite you to send me your favourite example of using a GADT to get the job done. Ideally I’d like to use examples that are (a) realistic, drawn from practice (b) compelling
and (c) easy to present without a lot of background. Most academic papers only have rather limited examples, usually eval :: Term t -> t, but I know that GADTs are widely used in practice.
Any ideas from your experience, satisfying (a-c)? If so, and you can spare the time, do send me a short write-up. Copy the list, so that we can all benefit.
Many thanks
Simon
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Hi Simon,
In the webcast I gave last week, I used a dumbed-down version of how we do safe queries in Persistent. The example is on slides 20-22 of [1]. Persistent works mostly the same way (and was inspired in this usage by groundhog[2]). If you want a more thorough explanation of how Persistent uses GADTs, let me know.
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):
data DocType = Doc1 | Doc2
data Element (doctype :: DocType) where
Paragraph :: Text -> Element doctype
Table :: [Element doctype] -> Element doctype
...
BulletList :: [[Element doctype]] -> Element Doc1
renderDoc1 :: Element Doc1 -> XML
renderDoc2 :: Element Doc2 -> XML
Michael