datatype T = A.{0..3} | B.Set({0,1}) | C definition of type A.0, B.{0}, B.{0,1}, C four uses of type
Datatypes may not be parameterised (T may not have arguments).
The datatype corresponds to the variant-record construct of
languages like Pascal.
At the simplest level it can be used to define a number of atomic
constants
datatype SimpleColour = Red | Green | Blue
but values can also be associated with the tags
Gun = {0..15}
datatype ComplexColour = RGB.Gun.Gun.Gun | Grey.Gun | Black | White
Values are combined with `.' and labelled using the appropriate tag, so that we could write
make_colour((r.g.b)@@x) = if r!=g or g!=b then RGB.x else if r==0 then Black else if r==15 then White else Grey.r
to encode a colour as briefly as possible.
Note that while it is possible to write
datatype SlowComplexColour = RGB.{r.g.b | r<-Gun, g<-Gun, b<-Gun} | ...
this is less efficient and the resulting type must still be rectangular, i.e., expressible as a simple product type. Hence it is not legal to write
datatype BrokenComplexColour = -- NOT RECTANGULAR
RGB.{r.g.b | r<-Gun, g<-Gun, b<-Gun, r+g+b < 128 } | ...
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