Slides for a short [talk at the Brisbane Functional Programming Group] BFPG, June 2012.

These are probably only useful if you need to jog your memory about something I said at the talk. If you really want to learn Template Haskell and QuasiQuotes, you should probably just head straight to the Template Haskell wiki page, and start following links from there.

Learning Template Haskell does require some detective work. Those resources which are up-to-date, namely the [relevant section of the GHC user manual] manual and the [Template Haskell library API documentation] API, are not intended as tutorials. And many of the resources which are useful for learning Template Haskell are either not altogether comprehensive, or somewhat out of date. But you're clever, so I'm sure you'll work it out!

A couple of questions came up which I didn't really address properly.

Nick asked whether quoting a name that is not in scope is an error. Indeed this is an error. TH quotes are lexically scoped, so a quoted name always refers to the name in scope at the point of the quotation, whether that name is itself lexically scoped, or a qualified global name. Global names add some subtleties, because they might not be imported in the scope where the quote is spliced. But the TH authors have thought about these things, and TH generally works as you'd expect it to.

It is possible to create a quoted AST with a "free variable" that can be captured by whatever name happens to be in scope at the point of splicing, but you have to go out of your way to do that explicitly. See the TH paper for the details.

Sanj asked what applications make use of Template Haskell and QuasiQuotes. I mentioned Yesod, since Ben's Yesod talk was up next, but here are a few more.

Derive: A library and tool for automatically deriving type-class instances. Uses reification to inspect the data type whose instances are to be derived, and provides ready-to-splice quoted declarations for the derived instances.

Data Accessor: A library for defining composable accessors for fields of data types. Comes with a Data Accessor Template package which uses reification to generate the accessors for a data type. The Lenses package provides similar capabilities, also using Template Haskell.

Boomerang: A library for defining invertible parsers. Uses Template Haskell to automatically derive some parser and pretty-printer combinators for a give data type.

For more examples, you can fetch the Hackage index and grep the cabal files for "TemplateHaskell" or "QuasiQuotes" (to find uses of the language extensions), or for "template-haskell" to find dependencies on the support libraries.