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function construction - Pattern that matches colors


Suppose I'm writing a function that takes a color as a parameter; for example:



drawShape[color_] := Graphics[{Style[Disk[], color]}];

But if the caller inputs an invalid color, bad things happen:


Bad


So I want to use a pattern to define drawShape only for values that are actually colors. Conceptually,


drawShape[color_Color] := ...

The problem is that unlike (say) Lists, Integers, Reals, Complexes, or Graphicses, color objects do not share a Color head. That is,


In[1]:= Red // Head
Out[1]= RGBColor

In[2]:= Hue[0.5] // Head
Out[2]= Hue
In[3]:= GrayLevel[0.5] // Head
Out[3]= GrayLevel
In[4]:= CMYKColor[0, 1, 1, 1/2] // Head
Out[4]= CMYKColor
In[4]:= Opacity[0.5, Purple] // Head
Out[4]= Opacity
In[5]:= Transparent // Head
Out[5]= GrayLevel


So that won't work. I also don't see any ColorQ function, with which I could write drawShape[color_ ? ColorQ] := ....


How can I write a pattern that matches any valid color object? Is there a more robust way than just testing for each of these heads?



Answer



Original method


colorQ = Quiet @ Check[Blend @ {#, Red}; True, False] &;

colorQ /@ {Red, Hue[0.5], GrayLevel[0.5], CMYKColor[0, 1, 1, 1/2], Opacity[0.5, Purple]}



{True, True, True, True, True}

colorQ /@ {17, 1.3, Pi, "not a color", {1, 2, 3}, Hue["bad arg"]}


{False, False, False, False, False, False}

You would use: drawShape[color_?colorQ] := . . .



Inspired by kguler's comment this might also be formulated as:



colorQ = Quiet[Head @ Darker @ # =!= Darker] &;

Or:


colorQ = FreeQ[Quiet @ Darker @ #, Darker] &;

Edit: Darker works on entire Image and Graphics objects and therefore the two forms immediately above will incorrectly return True in these cases. Blend solution is still valid.



Version 10 update and analysis


In version 10 there is a built-in function for this: ColorQ




ColorQ[color] yields True if color is a valid color directive and False otherwise.



A bit of spelunking reveals that the inner definition of this function is (contexts stripped for clarity):


iColorQ[args_?(ColorDirectiveQ[Head[#1[[1]]]] &), opts_] := 
  NumberQ[Quiet[ToColor[args[[1]], XYZColor][[1]]]]

This is very similar to my own method, however the inner definition of ColorDirectiveQ omits Opacity:


iColorDirectiveQ[args_, opts_] := 
 TrueQ[Quiet[
   MatchQ[args[[1]], 

    GrayLevel | RGBColor | CMYKColor | Hue | XYZColor | LUVColor | LABColor | LCHColor]]]

This means that the function will return False for e.g. Opacity[0.5, Purple] where mine returns True.


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