Am Donnerstag 18 Juni 2009 15:24:20 schrieb Jack Kennedy:
I think I understand the reason why, but still I find it disturbing that in this first expression, x has 6 elements:
Prelude> let x = [0,60..359]; y = [0,60..359] in (x, y, map degreesToRadians y)
You give no type for x, so the default is chosen, that is Integer. The expression map degreesToRadians y forces y to have type [Double] (you could give y a more general type if you specified a type signature).
([0,60,120,180,240,300],[0.0,60.0,120.0,180.0,240.0,300.0,360.0],[0.0,1.047 1975333333332,2.094395066 6666664,3.1415926,4.188790133333333,5.235987666666667,6.2831852])
But if I add a comparison to y, x now has 7 elements:
Prelude> let x = [0,60..359]; y = [0,60..359] in (x, y, map degreesToRadians y, *x==y*) ([0.0,60.0,120.0,180.0,240.0,300.0,360.0],[0.0,60.0,120.0,180.0,240.0,300.0 ,360.0],[0.0,1.0471975333 333332,2.0943950666666664,3.1415926,4.188790133333333,5.235987666666667,6.2 831852],True)
Now, the expression x==y forces x to have the same type as y, which still is [Double]. But, lo and behold: Prelude> let degreesToRadians :: Double -> Double; degreesToRadians d = d*pi/180 Prelude> let x :: (Num a, Enum a) => [a]; x = [0, 60 .. 359]; y :: (Num a, Enum a) => [a]; y = [0, 60 .. 359] Prelude> (x,y,map degreesToRadians y, x == y) ([0,60,120,180,240,300],[0,60,120,180,240,300], [0.0,1.0471975511965976,2.0943951023931953,3.141592653589793,4.1887902047863905,5.235987755982989,6.283185307179586],True)
I. J. Kennedy