RE: nice simple problem for someone struggling....
2007-12-21 17:08:28 GMT
Oh….
You are correct…
I thought from
“Num numberType => SquareConstructor
numberType”
We could deduce that (in English rather than get Haskell and FOL confusion)
all values of “SquareConstructor a”….the type of a would have be be in class Num?..
is this not correct?....if not….why not?
From:
d4ve.menendez <at> gmail.com [mailto:d4ve.menendez <at> gmail.com] On Behalf Of David Menendez
Sent: 21 December 2007 17:05
To: Nicholls, Mark
Cc: Jules Bean;
haskell-cafe <at> haskell.org
Subject: Re: [Haskell-cafe] nice
simple problem for someone struggling....
On Dec 21, 2007 11:50 AM, Nicholls, Mark <Nicholls.Mark <at> mtvne.com> wrote:
Now I have....
module Main where
data SquareType numberType = Num numberType => SquareConstructor
numberType
This is a valid declaration, but I don't think it does what you want it to. The
constraint on numberType applies only to the data constructor.
That is, given an unknown value of type SquareType a for some a, we do not have
enough information to infer Num a.
For your code, you want something like:
instance (Num a) => ShapeInterface (SquareType a) where
area (SquareConstructor side) = side * side
--
Dave Menendez <dave <at> zednenem.com>
<http://www.eyrie.org/~zednenem/ >
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