How can I construct (and destruct) masked images? about friday

[herrzinter]

The code compiles, if the type signature :: DelayedMask RGBPixel is added to the i' variable:

let i' = maskImage rgb :: DelayedMask RGBPixel

The problem arised as the fromFunction, and thus, the maskImage function are polymorphic ie. there type signature uses a type variable. At compile time, the compiler has to known the concrete type of this variable, thus, the type has to be specified explicitely or the compiler has to be able to infer the value. The type signature defines the type explicetly ie. it tells the compiler, that the application of the maskImage function results in a masked image of rgb pixels. Similiarly, if the convert function is used, the types have be instances of the corresponding typeclass, and the output type has to be specified by a type signature if it cannot be infered otherwise.