[Haskell] - How to use Complex? (Solved)

wuischke · 2009-12-13 13:53:22

Hi everyone,

I'm currently working through Write yourself a Scheme in 48 hours and doing the exercises.

One of the exercises is implementing a parser for the complex type. I found a (messy) solution when using two doubles for the real and imaginary part, but I can't figure out how to use Haskell's Complex type.

Simplified, the problematic code is the following:

data LispVal =  Ratio Rational
             | Complex Double Double

When I try to use "Complex" instead of "Double Double", I get the following error:

`Complex' is not applied to enough type arguments
Expected kind `?', but `Complex' has kind `* -> *'
In the data type declaration for `LispVal'

I don't understand above error message, but my understanding is that "Rational" is a type conversion (or something similar) from floating point for the class Ratio, whilst there's no such equivalent for the class Complex. How would I go on to write such a type conversion from two Double for Complex?

Prelude Complex Ratio> :t 0:+1
0:+1 :: (RealFloat t) => Complex t
Prelude Complex Ratio> :t (0.5 :: Rational)
(0.5 :: Rational) :: Rational
Prelude Complex Ratio> :t (0 % 2)
(0 % 2) :: (Integral t) => Ratio t

Last edited by wuischke (2009-12-13 19:37:59)

jeneshicc · 2009-12-13 14:18:46

Try this code.

data LispVal = Ratio Rational | Complex Double

VirtualRider · 2009-12-13 16:53:55

Complex is the "constructor" of LispVal

let l1 = Complex 5.0 5.0
:info l1

EDIT: Maybe this will help a little more:

test.hs:

data LispVal =  Ratio Rational | Complex Double Double deriving (Show)

l1 = Ratio 2
l2 = Complex 5.0 3.0


testLisp :: LispVal -> String
testLisp (Ratio x) = "I'm rational:" ++ show x
testLisp (Complex x y) = "We're complex: " ++ show x ++ " " ++ show y

ghci:

*Main> :load test.hs
*Main> testLisp l1
"I'm rational:2 % 1"
*Main> testLisp l2
"We're complex: 5.0 3.0"

Last edited by VirtualRider (2009-12-13 17:15:22)

wuischke · 2009-12-13 17:34:41

Thanks a lot for your answers. I'm afraid I may have posed my question insufficiently and stripped down too much, sorry. My code works fine when I represent a complex number as two doubles, I've written a Parsec parser and a showVal function, both of which work fine.

On the other hand, the Haskell library already contains a Complex type, see http://haskell.org/onlinereport/complex.html. I want to use this complex type instead of a tuple of two Doubles to represent a complex number. The infix constructor is :+, so <num1> :+ <num2> creates such a complex number.

I've tried:

module Main where
import Complex

data DerivedComplex = DComplex Complex

Result:

test.hs:4:31:
    `Complex' is not applied to enough type arguments
    Expected kind `?', but `Complex' has kind `* -> *'
    In the data type declaration for `DerivedComplex'

Also the infix constructor:

data DerivedComplex = DComplex :+

Result:

test.hs:5:0: parse error (possibly incorrect indentation)

And the constructor as it is written in above linked library report:

data DerivedComplex = DComplex Complex((:+))

Result:

test.hs:4:39: Not in scope: type constructor or class `:+'

How would I tell Haskell that my LispVal type may contain Complex numbers as found in this module?

VirtualRider · 2009-12-13 17:53:35

It's defined as "data (RealFloat a) => Complex a" so you have to specify the content-type:

data DerivedComplex = DComplex (Complex Double)

let d1 = DComplex (5.0 :+ 3.0)

wuischke · 2009-12-13 18:22:13

Wow, thanks for your fast answer, this solved indeed my problem. (And caused a couple new ones, but those I'll try to fight alone first.)

Edit: Thanks to you, there's now finally an acceptable answer for Exercise 7 in Section 3: http://en.wikibooks.org/wiki/Write_Your … Exercise_7

Last edited by wuischke (2009-12-13 18:36:36)

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#1 2009-12-13 13:53:22

[Haskell] - How to use Complex? (Solved)

#2 2009-12-13 14:18:46

Re: [Haskell] - How to use Complex? (Solved)

#3 2009-12-13 16:53:55

Re: [Haskell] - How to use Complex? (Solved)

#4 2009-12-13 17:34:41

Re: [Haskell] - How to use Complex? (Solved)

#5 2009-12-13 17:53:35

Re: [Haskell] - How to use Complex? (Solved)

#6 2009-12-13 18:22:13

Re: [Haskell] - How to use Complex? (Solved)

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