[Haskell-cafe] Pattern variables in Template Haskell

[Alejandro Serrano Mena]

Dear Haskell-cafe,
I'm trying to use Template Haskell to generate patterns in Haskell code,
but after a couple of days of trying, I have not come with a solution. I
hope somebody could help me.

My problem is basically that I want something like:

example :: Maybe a -> a
example $(j "x") = x

to work correctly. The idea of this simple quoter is to generate code
similar to:

example :: Maybe a -> a
example (Just x_123) = x_123

that is, to create the "Just" pattern, and to make "x" bind to the same
element. Right now my code reads:

j :: String -> Q Pat
j s = return (ConP 'Just [VarP (mkName s)])

which generated a new variable "x" each time, thus unable to match with the
"x" in the RHS. I have also tried with lookupValueName.

Note that my problem is a bit more complicated, with variable names coming
from a different source. However, I think that if I solve this problem
where I can make variables in a pattern and in an expression match, I
should be able to use them in my complete solution.

Thanks in advance.
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