#3788: Improved message for GHC "No instance for" Errors
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Reporter: rhapsodyv | Owner:
Type: proposal | Status: new
Priority: normal | Component: Compiler
Version: 6.10.4 | Keywords: errors
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Hi,
I would like to propose a modification in the ghc message for "no
instance for" errors.
In the message, the ghc tells that there's no instance for the type
you are using and it give you a hint that you can fix it adding a new
instance for your type. But it don't tell me any instance alternative
that already exists. So, if I don't know the correct type, I need to
look at the documentation.
At least for me, I get this message when I'm using the function in the
wrong way or when I forget to specify the some function signature.
It's quite rare the I want to add a new instance.
Of course that it's not a problem to look at the documentation, but
improving it can save a significant time and help the beginers to
guess the error and how fix it.
In C++ and Java, when I use g++ or javac and I make a wrong
overloading, the compiler show me some alternatives. I think it help
more than tell me to add a new overload to the function.
It's possible to do it in ghc?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3788>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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