Tomasz Zielonka wrote:
voidcast v@(VariantWithTwo{}) = v { field1 = Void , field2 = Void } voidcast v@(VariantWithOne{}) = v { field1 = Void } I would not expect that updating only field1 can change the type of v.
But it can. Note that if you change the second field1 to field3 (both in datatype definition and voidcast function), the code will be accepted.
The right thing is to construct a new value.
In the report, section 3.15.3, "Updates Using Field Labels", the translation is simply a pattern-match and (re-)construction. There is no requirement that "input" and "output" types are the same. It can be a nice feature actually, I've used it once or twice.
Obviously this (magic) reconstruction does not work always (and not in the given case). Even consecutive updates (that could be recognized by the desugarer) don't work: voidcast v@VariantWithTwo{} = v { field1 = Void} {field2 = Void } I think, the type checker of programatica requires that input and output type of updated values are the same (not only because comma-separated field updates are desugared to consecutive updates.) Christian