Malcolm Wallace wrote:
> I believe this is a very different question from the consecutive
> update one.
I agree, consecutive and parallel updates are quite different.
> I can see no
> reason to outlaw a type change, where all the relevant types change
> at the same time:
>
> update_ok v@(A{}) = v { one=Void, two=Void }
> update_ok' v@(A{}) = (\A one two -> A Void Void) v
Because typing fails in your original example:
voidcast [EMAIL PROTECTED] = v {field1 = Void}
What would be the "lambda equivalent"?
> voidcast [EMAIL PROTECTED] =
(\ (VariantWithOne a) -> VariantWithOne Void) v
would go through. Whereas
> voidcast [EMAIL PROTECTED] = ( \ x -> case x of
VariantWithTwo a b -> VariantWithTwo Void b
VariantWithOne a -> VariantWithOne Void) v
fails like the field notation.
I think writing "VariantWithOne {field1 = Void}" instead of
"v {field1 = Void}" is as clear as writing "Right r" again on the rhs
instead of "x" from the pattern "x@(Right r)"
Christian
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