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Today's Topics:
1. why does the superclass not work? (Alexander Chen)
2. Re: why does the superclass not work? (Mihai Maruseac)
----------------------------------------------------------------------
Message: 1
Date: Wed, 29 Apr 2020 21:34:35 +0200 (CEST)
From: Alexander Chen <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] why does the superclass not work?
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"
Hi,
superclass.hs
myX = 1 :: Int
sigmund :: Int -> Int
sigmund x = myX
prelude> :l superclass.hs
[1 of 1] Compiling Main
Ok, one module loaded.
==================================================================================
superclass.hs
myX = 1 :: Int
sigmund :: Num -> Num
sigmund x = myX
prelude> :l superclass.hs
typed_checked.hs:3:13: error:
• Expecting one more argument to ‘Num’
Expected a type, but ‘Num’ has kind ‘* -> Constraint’
• In the type signature: sigmund :: Num -> Num
|
3 | sigmund :: Num -> Num | ^^^
typed_checked.hs:3:20: error:
• Expecting one more argument to ‘Num’
Expected a type, but ‘Num’ has kind ‘* -> Constraint’
• In the type signature: sigmund :: Num -> Num
===================================================================================
I would think since Num is a superclass of Int defining the type in the
superclass would be OK, also in the error message it refers to (see black) what
does that refer to?
thanks!
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Message: 2
Date: Wed, 29 Apr 2020 13:11:54 -0700
From: Mihai Maruseac <[email protected]>
To: Alexander Chen <[email protected]>, The Haskell-Beginners
Mailing List - Discussion of primarily beginner-level topics related
to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] why does the superclass not work?
Message-ID:
<CAOMsUMLRRqR6LEFQAwMcisB-tfJs2HJGs6=+pg2wy6h7xtg...@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
Hi.
Num is not a superclass of Int. Int is a type whereas Num is a type of a type.
The fix would be
sigmund :: Num a => a -> a
...
This signature means "sigmund has type a to a where a is constrained
to be a type variable of type Num" (so Num is the type of the type
variable a)
On Wed, Apr 29, 2020 at 12:41 PM Alexander Chen <[email protected]> wrote:
>
> Hi,
>
> superclass.hs
>
> myX = 1 :: Int
>
> sigmund :: Int -> Int
> sigmund x = myX
>
>
> prelude> :l superclass.hs
> [1 of 1] Compiling Main
> Ok, one module loaded.
>
> ==================================================================================
>
> superclass.hs
>
> myX = 1 :: Int
>
> sigmund :: Num -> Num
> sigmund x = myX
>
> prelude> :l superclass.hs
> typed_checked.hs:3:13: error:
> • Expecting one more argument to ‘Num’
> Expected a type, but ‘Num’ has kind ‘* -> Constraint’
> • In the type signature: sigmund :: Num -> Num
> |
> 3 | sigmund :: Num -> Num | ^^^
>
> typed_checked.hs:3:20: error:
> • Expecting one more argument to ‘Num’
> Expected a type, but ‘Num’ has kind ‘* -> Constraint’
> • In the type signature: sigmund :: Num -> Num
>
>
> ===================================================================================
>
> I would think since Num is a superclass of Int defining the type in the
> superclass would be OK, also in the error message it refers to (see black)
> what does that refer to?
>
> thanks!
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
--
Mihai Maruseac (MM)
"If you can't solve a problem, then there's an easier problem you can
solve: find it." -- George Polya
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