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Type question

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  Type question Black Swan <root@rhok.xyz> - 2018-07-16 18:02 +0000
    Re: Type question Mark Carroll <mtbc@bcs.org> - 2018-07-16 20:56 +0100
    Re: Type question Mark Carroll <mtbc@bcs.org> - 2018-07-16 21:05 +0100
      RE: Re: Type question Black Swan <root@rhok.xyz> - 2018-07-17 15:04 +0000
        Re: Type question Mark Carroll <mtbc@bcs.org> - 2018-07-17 16:11 +0100

#488 — Type question

From:
Black Swan 
Subject:
Haskell question
Date:
07.02.2106 17:28
Newsgroups:
comp.lang.functional


I'm struggling with the following function:

The following declaration works:

data DividedResult = Result Integer | DividedByZero deriving (Eq, Show)

dividedBy num denom =
let
(count,remainder,success) = x num denom 0
in if success then (Result count) else DividedByZero
where
x num denom count
| denom == 0 = (0,0,False)
| num | otherwise = x (num - denom) denom (count + 1)


When I check the type of dividedBy, I get 

*Lib> :t dividedBy 
dividedBy :: (Num b, Ord b) => b -> b -> DividedResult

However, if I try to add the type declaration line:

dividedBy :: (Num a, DividedResult b) => a -> a -> b

The module fails to load, yielding the following error:

 Expected a constraint, but DividedResult has kind *
 In the type signature:
dividedBy :: (Num a, DividedResult b) => a -> a -> b
|
10 | dividedBy :: (Num a, DividedResult b) => a -> a -> b
| ^^^^^^^^^^^^^
Failed, no modules loaded.

Could someone please kindly point out my mistake and suggest a type declaration line that works?

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#489

On 16 Jul 2018, Black Swan wrote:

> I'm struggling with the following function:
>
> The following declaration works:

Don't know what happened to your whitespace here:

> data DividedResult = Result Integer | DividedByZero deriving (Eq, Show)
>
> dividedBy num denom =
> let
> (count,remainder,success) = x num denom 0
> in if success then (Result count) else DividedByZero
> where
> x num denom count
> | denom == 0 = (0,0,False)
> | num | otherwise = x (num - denom) denom (count + 1)

I can't quite make sense of this even if I try to correct the indents.
What's going on with the num guard bit?

If your news client is causing problems, happy to take a look at this
from some URL instead.

-- Mark

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#490

On 16 Jul 2018, Black Swan wrote:

> data DividedResult = Result Integer | DividedByZero deriving (Eq, Show)
(snip)
> *Lib> :t dividedBy 
> dividedBy :: (Num b, Ord b) => b -> b -> DividedResult
>
> However, if I try to add the type declaration line:
>
> dividedBy :: (Num a, DividedResult b) => a -> a -> b
(snip)
> Could someone please kindly point out my mistake and suggest a type
> declaration line that works?

Beyond the issues with the code as presented, DividedResult is just an
algebraic datatype. The constraint syntax you're using in the type
signature is for classes, not types: classes simply limit the types that
can be used to fill in the type variables like a and b. Also you do need
the Ord because that's what "<" needs. It might help if you back up and
let us know what you're hoping this type signature will achieve that the
inferred one doesn't. It may help to note that in many places where in
C++ or whatever you might use a thing called a class, in Haskell it will
still simply be a type, with classes being at another step removed and
not always needed in the same way.

-- Mark

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#491

On Mon Jul 16 21:05:06 2018 Mark Carroll  wrote:
> On 16 Jul 2018, Black Swan wrote:
> 
> > data DividedResult = Result Integer | DividedByZero deriving (Eq, Show)
> (snip)
> > *Lib> :t dividedBy 
> > dividedBy :: (Num b, Ord b) => b -> b -> DividedResult
> >
> > However, if I try to add the type declaration line:
> >
> > dividedBy :: (Num a, DividedResult b) => a -> a -> b
> (snip)
> > Could someone please kindly point out my mistake and suggest a type
> > declaration line that works?
> 
> Beyond the issues with the code as presented, DividedResult is just an
> algebraic datatype. The constraint syntax you're using in the type
> signature is for classes, not types: classes simply limit the types that
> can be used to fill in the type variables like a and b. Also you do need
> the Ord because that's what "<" needs. It might help if you back up and
> let us know what you're hoping this type signature will achieve that the
> inferred one doesn't. It may help to note that in many places where in
> C++ or whatever you might use a thing called a class, in Haskell it will
> still simply be a type, with classes being at another step removed and
> not always needed in the same way.
> 
> -- Mark

Thanks Mark, much appreciated!

I am starting to get the hang of it. Just figured out that the declaration that I needed was:

dividedBy :: (Num n, Ord n) => n -> n -> DividedResult

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#492

On 17 Jul 2018, Black Swan wrote:

> On Mon Jul 16 21:05:06 2018 Mark Carroll  wrote:
>> On 16 Jul 2018, Black Swan wrote:
(snip)
>> > *Lib> :t dividedBy 
>> > dividedBy :: (Num b, Ord b) => b -> b -> DividedResult
(snip)
> I am starting to get the hang of it. Just figured out that the declaration
> that I needed was:
>
> dividedBy :: (Num n, Ord n) => n -> n -> DividedResult

Glad to hear it. Kudos to the type inference system then. (-:

-- Mark

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