[Template-haskell] quasiquoting and guards

[Claus Reinke]

> I'd like to use quasiquotation to emulate bash's case statement in
> Haskell.  That is, I'd like to turn
> 
> example x = case x of
>  [$rx|.*foo.*] -> "contains foo"
>  _             -> "doesn't contain foo"

Assuming that regexqq produces (String -> Maybe [String]), and
works rather better for expressions than for patterns, you could 
simply lift mplus to functions, and define your own caseRE (your
example doesn't really make use of case/pattern-matching):

    import Control.Monad
    import Data.Char
    import Data.List

    (matchA .|. matchB) x = matchA x `mplus` matchB x

    (regex .->. rhs) x =  regex x >> return rhs -- (or >>=, if prefered)

    caseRE x matches otherwise = maybe otherwise id (matches x)

    a x = Just [this,that]
      where (this,that) = span isSpace x
    b x = Nothing

    test = caseRE "hello world" 
             (   (b .->. "hi")
             .|. (a .->. "ho"))
                 "otherwise"

Then you can either fiddle with operator priorities, or drop
the thin facade and use MonadPlus directly;-)

You might also want to be able to refer to match results in
the rhs, no matter whether that is possible in bash - this would
seem to need control over both lhs and rhs anyway.

Hth,
Claus
 
> into (assuming an appropriate match :: String -> String -> Bool)
> 
> example x = case x of
>  s | match ".*foo.*" s -> "contains foo"
>  _                     -> "doesn't contain foo"
> 
> But it seems like I can't do so, because despite appearances, the
> difference between the two cases in the second example is not in
> "what's to the left of ->".  Instead, the first has a GuardedB body
> while the second has a NormalB body.  (Having reread the Haskell
> Report, I now understand why this is, but it was surprising at first.)
> Whatever my definition of the quasiquoter rx is, the first example is
> going to expand to something with two NormalB bodies, so I can't
> achieve the desired expansion.
> 
> Is there a clever workaround?  I tried (ab)using view patterns, but
> they're not yet supported by Template Haskell.  
> 
> Regards,
> Reid Barton
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