Hi Ben,
With ghc-7.8.4 I get a different error "Empty record update of:
default_config" when using a wildcard to update a record.
I think you can't use wildcards in record updates because it's harder
(for users and for ghc) to figure out which fields are involved when
you don't name a constructor. If you don't mind naming the constructor
twice, you can do an "update" with:
myC = case defC of
C { .. } -> C { .. }
where a = 2
-- myC == C 2 2 3
-- where we have for example
data CType = C { a, b, c :: Int } deriving Show
defC = C 1 2 3
If CType had multiple constructors, I think you'd be better off doing
the update with NamedFieldPuns syntax (defC { a }), or ordinary update
syntax (defC { a = 2 }).
Regards,
Adam
Regards,
Adam
On Tue, Feb 24, 2015 at 9:05 PM, Ben Franksen
I just noted that code like
my_config = default_config {..} where name = "my project" description = "some longer text"
gives me a syntax error, even if I have NamedFieldPuns and RecordWildCards extensions enabled. It seems that these extensions only work for record constructors and not for updating values.
Is there a special reason puns/wildcards are not allowed in record updates?
Cheers Ben -- "There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult." ― C.A.R. Hoare
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