color space conversion

Recently I was working on some C code for an image-manipulation app, and I had to write logic to convert between RGB and HSL (hue-saturation-lightness) representations of color. For extra confidence in its correctness, I decided to cross-check the C version against this Haskell implementation.

	hsl :: (Int, Int, Int) -> (Double, Double, Double)
	hsl (r',g',b') = r' == g' && g' == b' ? ((0,0,l), (h,s,l))
	where
	[r,g,b] = map ((/ 255) . fromIntegral) [r',g',b']
	cmin = minimum [r,g,b]
	cmax = maximum [r,g,b]
	d = cmax - cmin
	l = (cmin + cmax) / 2
	s = l > 0.5 ? (d / (2 - cmax - cmin), d / (cmax + cmin))
	h = h' < 0 ? (h' + 1, h')
	h' | r' == maximum [r',g',b'] = dB - dG
	| g' == maximum [r',g',b'] = (1 / 3) + dR - dB
	| otherwise = (2 / 3) + dG - dR
	where
	dR = (((cmax - r) / 6) + (d / 2)) / d
	dG = (((cmax - g) / 6) + (d / 2)) / d
	dB = (((cmax - b) / 6) + (d / 2)) / d
	--------------------------------------------------------------
	rgb :: (Double, Double, Double) -> (Int, Int, Int)
	rgb (h,s,l)
	| abs (0 - s) < 0.001 = f l l l
	| otherwise = f r g b
	where
	q = l < 0.5 ? (l * (1 + s), l + s - l*s)
	p = 2*l - q
	r = hueToRgb p q $ h + 1/3
	g = hueToRgb p q h
	b = hueToRgb p q $ h - 1/3
	f x y z = (round $ x*255, round $ y*255, round $ z*255)
	hueToRgb p q h''
	| h < 1/6 = p + (q - p) * 6 * h
	| h < 1/2 = q
	| h < 2/3 = p + (q - p) * 6 * (2/3 - h)
	| otherwise = p
	where
	h' = h'' < 0 ? (h'' + 1, h'')
	h = h' > 1 ? (h' - 1, h' )
	p ? (a,b) = if p then a else b; infix 2 ?

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