Re: Record Puns/Wildcards

adam vogt wrote:

With ghc-7.8.4 I get a different error "Empty record update of: default_config" when using a wildcard to update a record.

Yes. I remembered it wrong, this is the error I get, too (with ghc-7.6.3).

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.

Thanks, I think I see the problem now. If there was more than one constructor it could have completely different fields. So, in case of multiple constructors this would be a bad idea. We could make an exception for data types with a single constructor, but perhaps it doesn't make sense to provide extra sugar for this special case.

If you don't mind naming the constructor twice, you can do an "update" with:

myC = case defC of C { .. } -> C { .. } where a = 2

Nice trick, but I actually do mind the duplication ;-) The reason I asked about this is that I'd like to have a simple way to embed user configuration, preferably without braces and commas and without having to define each possible configuration value (i.e. only those fields that differ from the default). Like xmonad, yi, etc, but simpler and more accessible to non-Haskell-programmers. Really off-topic but on a related note, I never understood why the named field syntax in Haskell does not use semicolon as a separator (instead of comma) and allow indentation to replace braces and separators, as in data X = X field1, field2 :: String field3 :: Int and similar for lists and tuples. Cheers Ben

On Tue, Feb 24, 2015 at 9:05 PM, Ben Franksen wrote:

...

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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Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users -- "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