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?

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

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

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

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

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

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

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

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

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)