#4940: Bad error message using poly pat bind with MonoPatBinds
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Reporter: batterseapower | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: 7.2.1
Component: Compiler (Type checker) | Version: 7.0.1
Resolution: | Keywords:
Testcase: | Blockedby:
Difficulty: | Os:
Unknown/Multiple
Blocking: | Architecture:
Unknown/Multiple
Failure: Incorrect warning at compile-time |
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Changes (by MartijnVanSteenbergen):
* cc: martijn@… (added)
Comment:
I was going to report something similar but found this bug here, so will
add myself to CC. The error message in GHC 7.0.3 is still cryptic; it took
a while to figure out what the problem and the solution were. I found bug
2187 which has a longer discussion and has a broken link to the Haskell'
page on this issue; the new link is http://hackage.haskell.org/trac
/haskell-prime/wiki/MonomorphicPatternBindings
My use case for -XNoMonoPatBinds is this:
Like yallop in bug 2187, I am using TH to derive tuples with polymorphic
elements. The size of the tuple returned is equal to the number of
constructors in the datastructure the functionality is derived for. I'm
not a big fan of TH functions generating and introducing names into the
top-level scope because it breaks modularity. Top-level pattern bindings
are a very attractive solution: they allow me to give the user full
control over the naming. It also allows the user to ignore a specific
element of the tuple if it's undesired, by using a wildcard.
Here is an example:
https://github.com/MedeaMelana/JsonGrammar/blob/aad158899d9845f7b15a65609a818e557f260dcf/Example.hs
I understand the reason for MonoPatBinds, but I will miss them. :-) :-( Is
there another way in which I can let the user handle the naming of
multiple generated polymorphic values this easily? The only thing I can
think of is to add an argument for the list of to-be-introduced names to
the TH function.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4940#comment:4>
GHC <http://www.haskell.org/ghc/>
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