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TIL Why AND should have two representations.

Kasey Speakman
collector of ideas. no one of consequence.
・1 min read

In most languages, the concept AND has two different representations. For example, in F# there is logical AND:

let awesome = good && great
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Then there is bitwise AND:

// this is pretty common when working with Flags
let hasRed = someColors &&& Color.Red = Color.Red
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But then there's VB, for which both concepts use the keyword And. So I implemented a feature in VB to clone an image. I had this code snippet.

    Public Shared Function CloneImage(ByVal sourceImage As Bitmap) As Bitmap
        If sourceImage.PixelFormat And PixelFormat.Indexed <> 0 Then
            Return CreateIndexedImage(sourceImage)
            Return CreateNonIndexedImage(sourceImage)
        End If
    End Function
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This code always calls CreateIndexedImage. And it took me a bit to figure out why. It was because the language sees the keyword And as the lowest priority operator and evaluates it last. It does that because it is only considering the priority of logical And. So the actual evaluation was something like (sourceImage.PixelFormat) And (PixelFormat.Indexed <> 0). VB coerces the type of PixelFormat to be truthy, and the second condition will always be true.

The fix was to add parenthesis

        If (sourceImage.PixelFormat And PixelFormat.Indexed) <> 0 Then
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But this could have been avoided by simply having a different operator between the two uses. Then each could have a different order of precedence.

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