Hi all,
I don’t understand why this code: 50: instance (IsNat n, RealFloat a) => FFT (RTree n) a where is generating this error: fft_test.hs:50:36: Illegal instance declaration for ‘FFT (RTree n) a’ (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use FlexibleInstances if you want to disable this.) In the instance declaration for ‘FFT (RTree n) a’ and was hoping someone could help me get it. Thanks! -db _______________________________________________ Haskell-Cafe mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe |
From the Haskell 2010 report: "An instance
declaration introduces an instance of a class. Let class cx => C u where { cbody } be a class
declaration. The general form of the corresponding instance
declaration is: instance cx′ => C (T u_{1} … u_{k}) where { d } where k ≥ 0."
Note that your instance declaration has a form C (T u1) u2. I think adding the FlexibleInstances extension, as suggested by the compiler, will work. Although, depending on the context, you might need UndecidableInstances too. The core problem is that the FFT class must have been defined with the MultiParamTypeClasses extension, which usually leads to nastiness like this. On 11/10/2015 11:15 AM, David Banas
wrote:
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In reply to this post by David Banas-2
Hi David,
Andrey's answer is correct. I asked a similar question on StackOverflow a few years back [1]. You might find the resulting discussion useful. Chris [1] http://stackoverflow.com/q/8633470/617159 On Wed, Nov 11, 2015 at 5:15 AM, David Banas <[hidden email]> wrote: > Hi all, > > I don’t understand why this code: > > 50: instance (IsNat n, RealFloat a) => FFT (RTree n) a where > > is generating this error: > > fft_test.hs:50:36: > Illegal instance declaration for ‘FFT (RTree n) a’ > (All instance types must be of the form (T a1 ... an) > where a1 ... an are *distinct type variables*, > and each type variable appears at most once in the instance head. > Use FlexibleInstances if you want to disable this.) > In the instance declaration for ‘FFT (RTree n) a’ > > and was hoping someone could help me get it. > > Thanks! > -db > > > _______________________________________________ > Haskell-Cafe mailing list > [hidden email] > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > -- Chris Wong (https://lambda.xyz) "I had not the vaguest idea what this meant and when I could not remember the words, my tutor threw the book at my head, which did not stimulate my intellect in any way." -- Bertrand Russell _______________________________________________ Haskell-Cafe mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe |
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