[Haskell-beginners] Runtime error “Could not deduce (Integral Float) arising from a use of..”

[Jack Vice]

David and Kyle, Thank you both.  I declared the types and added a
'fromIntegral' and got things working!


On Thu, Aug 23, 2018 at 8:35 AM David McBride <toad3k at gmail.com> wrote:

> Floating is the type class of types that are floating point.  The two most
> common instances are Float and Double.  Integral is the class of integer
> types.  Most commonly Int and Integer.
>
> stdDev
>   :: (Floating a1, Floating a, Integral a1) =>
>
> When you see a type like that it means type a1 is both a Floating and also
> an Integral.  Intellectually that is impossible, but as far as ghc is
> concerned there could be a type that is an instance of both classes, so it
> allows it.  But when you try to call it, there's no type in scope that you
> can affix to a1 to please it, so it will always error.
>
> The reason it has both Floating and Integral on the same type is that you
> are using several functions on various arguments of your function that
> imply that the types of those arguments must be instances of various
> classes.
>
> fromIntegral :: (Integral a, Num b) => a -> b
> sumOfMinusMeans :: (Eq t, Floating t) => Int -> t -> [[t]] -> t
> sqrt :: Floating a => a -> a -- (this function may not be a factor)
>
> I strongly recommend you write out a type for stdDev, but fix a1 to a
> concrete type and then ghc can tell you why that type won't work.
> stdDev :: Int -> Int -> [Double] -> [[Double]] -> [Double]
>
>     • No instance for (Integral Double)
>         arising from a use of ‘fromIntegral’
>
> stdDev :: Int -> Int -> [Integer] -> [[Integer]] -> [Double]
>
>     • No instance for (Floating Integer)
>         arising from a use of ‘sumOfMinusMeans’
>
> And then think really hard about what types you want stdDev to accept.
> Rework its the definition until the compiler is happy.  I suspect it is an
> extraneous fromIntegral.
>
> On Thu, Aug 23, 2018 at 6:26 AM, Jack Vice <jack.vice at gmail.com> wrote:
>
>> I am trying to calculate the standard deviation for each index in a list
>> of lists of floats. Using Ubuntu 18.04, ghc 8.0.2. I am getting the
>> following runtime error which I have googled and still don't understand
>> what "Integral Float" is exactly or even which parameter is causing the
>> trouble.
>>
>> *Main> let z = stdDev 0 2 y x
>> <interactive>:250:9: error:
>> • Could not deduce (Integral Float) arising from a use of ‘stdDev’
>>   from the context: Floating a
>>     bound by the inferred type of z :: Floating a => [a]
>>     at <interactive>:250:5-38
>> • In the expression: stdDev 0 (length (head (x))) y x
>>   In an equation for ‘z’: z = stdDev 0 (length (head (x))) y x
>>
>> Code:
>>
>> -- i is start index, l is length of each list, ms is list of means,
>> --    xs is Matrix
>> stdDev i l ms xs
>>      | i < l     = sqrt(fromIntegral(sumOfMinusMeans i (ms!!i) xs) /
>>                              fromIntegral(l)):(stdDev (i+1) l ms xs)
>>      | otherwise = []
>>
>> --i is index, m is mean for the index
>> sumOfMinusMeans i m (x:xs)
>>      | xs == []     = (x!!i - m)**2
>>      | i < length x = (x!!i - m)**2 + (sumOfMinusMeans i m xs)
>>      | otherwise    = 0
>>
>> Types:
>>
>> *Main> :t stdDev
>> stdDev
>>   :: (Floating a1, Floating a, Integral a1) =>
>>      Int -> Int -> [a1] -> [[a1]] -> [a]
>>
>> *Main> :t sumOfMinusMeans
>> sumOfMinusMeans :: (Eq t, Floating t) => Int -> t -> [[t]] -> t
>>
>> Variables:
>>
>> *Main> y
>> [380.0,1.0]
>> *Main> x
>> [[600.0,1.0],[400.0,1.0],[170.0,1.0],[430.0,1.0],[300.0,1.0]]
>>
>>
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