[Haskell-cafe] Question, re: illegal instance error.

Chris Wong lambda.fairy at gmail.com
Wed Nov 11 06:33:11 UTC 2015

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.


[1] http://stackoverflow.com/q/8633470/617159

On Wed, Nov 11, 2015 at 5:15 AM, David Banas <capn.freako at gmail.com> 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
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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

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