The *splicing* of patterns is considered tricky, see:
http://hackage.haskell.org/trac/ghc/ticket/1476
Implementing pattern quotations might be less tricky, but I would
imagine to make them useful, you'd have to allow splicing things
*into* them, which requires implementing pattern splicing. That's my
non-expert take on the issue.
On Wed, Feb 3, 2010 at 7:32 PM, Brad Larsen
I'll put my question at the very front of this message, and then give the context.
My question: Are Template Haskell pattern quotations (i.e., [p| ... |]) not implemented in GHC because they are rather tricky to implement, or because there has not been demand for them?
And now, the context.
I am working on an embedded DSL/compiler in Haskell, and I want to make use of the recently-much-discussed quasiquoting mechanism to ease reading & writing various transformations.
I read through the quasiquoting paper [1] again and played around with some simple examples. In the paper, Data.Generics is used so that the same parser can be used for quasiquoting both expressions and patterns. This requires adding constructors to the ASTs being quasiquoted for antiquotations to be possible.
In my application, I don't anticipate using Data.Generics for parser reuse, for a few reasons: * I haven't read the SYB papers, and don't understand how Data.Generics or Data.Data work. * My ASTs in some cases involve GADTs with several phantom type parameters (is that the right terminology?), and DeriveDataTypeable does not work with them. * I don't want to add the extra constructors necessary to support antiquotation with the Data.Generics approach.
So, I'm stuck writing separate ExpQ / PatQ parsers. (Actually, I can write the parser once, if it takes a dictionary of semantic actions as a parameter, in which case I only need to write the dictionary of ExpQ actions and the dictionary of PatQ actions.) When writing the ExpQ parser, I can use Template Haskell expression quotations, [| ... |]. When writing the PatQ parser, I have to resort to using the various pattern construction combinators, which is unfortunate! I'd like to be able to use pattern quotations instead.
So, my question once again: How hard would it be to implement the pattern quotations from the Template Haskell paper in GHC?
Sincerely, Brad
References:
[1] Geoffrey B. Mainland. Why It's Nice to be Quoted: Quasiquoting for Haskell. http://www.eecs.harvard.edu/~mainland/ghc-quasiquoting/mainland07quasiquotin... _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users