On Wed, 16 Jul 2014 22:32:27 -0400, Leza Morais Lutonda
Hi,
Consider the following type classes:
class (Signal sx) => Complexable sx where type ComplexSignalType ....
class (Complexable sx) => FourierTransformable sx where fft :: (cx ~ ComplexSignalType sx) => cx -> cx ....
and an instance:
instance (RealFloat e) => Complexable [e] where type ComplexSignalType = [Complex e]
instance (RealFloat e) => FourierTransformable [e] where fft = ...
Why this example will give errors:
*DSP.Signals.Core Prelude Data.Complex> :t ax ax :: [Complex Double] *DSP.Signals.Core Prelude Data.Complex> fft ax
<interactive>:90:1: No instance for (FourierTransformable s0 [Complex Double]) arising from a use of ‘fft’ The type variable ‘s0’ is ambiguous Note: there is a potential instance available: instance [overlap ok] (RealFloat e, Math.FFT.Base.FFTWReal e) => FourierTransformable [e] [Complex e] -- Defined at src/DSP/Signals/Instances/List.hs:40:10 In the expression: fft ax In an equation for ‘it’: it = fft ax
Thanks!
The reason you're getting this error is because all you know about your instance is that the type of fft is :: [Complex Double] -> [Complex Double]. Since by definition this [Complex Double] is ComplexSignalType sx, all we know about the associated instance (parametrized by sx) is that ComplexSignalType sx ~ [Complex Double]. Since ComplexSignalType is a type family, it's not injective, so this does not suffice to uniquely identify our actual instance (sx, here called s0 by GHC). For example, we could have a second instance: instance Complexable () where type ComplexSignalType () = [Complex Double] instance FourierTransformable () where fft = error "oops" And with this instance, fft would also be [Complex Double] -> [Complex Double] but the behavior of would clearly be different to your actually intended function. If you think your signal function is bijective, you could use a two-way functional dependency like this: class Complexable sx cx | sx -> cx, cx -> sx class Complexable sx cx => FourierTransformable sx cx where fft :: cx -> cx Or if you want to avoid the overhead of redesigning Complexable you could even do something like this: class (cx ~ ComplexSignalType sx, Complexable sx) => FourierTransformable sx cx | cx -> sx where fft :: cx -> cx Either way, if instance selection should be possible based solely on the type of ‘cx’, then ‘cx’ must imply everything contained in the instance head.