[Haskell-cafe] Read Instance for UArray won't port to linux

Bulat Ziganshin bulat.ziganshin at gmail.com
Wed Mar 14 16:45:37 EDT 2007

Hello SevenThunders,

Wednesday, March 14, 2007, 10:32:23 PM, you wrote:

the type variables are dark side of GHC, and you need to have at least
1 mlOleg of brain to understand them. it will be great if someone will
ever write reasonable introduction into this. meanwhile, you can look
into ghc docs

> You guys are awesome!  I post this not 12 hours ago and I already have a
> complete treatise on the subject.  Yeah to clarify things putting an
> ellipsis between b and c would help.  But also clarify the meaning of
> distinct type variables.  Does this mean the type variable must not be
> parameterized?

> I ran into this because I decided during my port that I would try to learn
> some of the better build tools on linux.   So now I'm acquainted with Cmake,
> which is a great tool and cabal which is also very impressive. My problem
> boiled down to the fact that I didn't know how to set the correct compiler
> flags within cabal.  I figured out the FFI flags and now I suppose the gch
> extensions can be set with Ghc-options: -fglasgow-exts in my .cabal file. Is
> there a type in the Extensions field that corresponds to this?

> Spencer Janssen-2 wrote:
>> It looks like you forgot to pass a compiler flag, namely -fglasgow-exts.
>> Cheers,
>> Spencer Janssen
>> On Tue, 13 Mar 2007 22:20:20 -0700 (PDT)
>> SevenThunders <mattcbro at earthlink.net> wrote:
>>> I have the pleasure of porting a good sized Haskell application to
>>> linux. So far the Haskell code has compiled without incident, however
>>> some code that I hacked
>>> to implement a Read instance for Unboxed Arrays does not compile on
>>> linux even though it compiles just fine on Windows XP in Haskell 6.6.
>>> The code reads as,
>>> instance   Read (UArray Int Double)  where
>>>     readsPrec p = readParen (p > 9)
>>>            (\r -> [(array b as :: UArray Int Double, u) | ("array",s)
>>> <- lex r,
>>>                                      (b,t)       <- reads s,
>>>                                      (as,u)      <- reads t   ])
>>> The error in linux is:
>>>     Illegal instance declaration for `Read (UArray Int Double)'
>>>         (The instance type must be of form (T a b c)
>>>          where T is not a synonym, and a,b,c are distinct type
>>> variables) In the instance declaration for `Read (UArray Int Double)'
>>> Why does it want three parameters for the instance type?  I am
>>> baffled by this.
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Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com

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