[Haskell-cafe] default values in a record structure

[Sebastiaan Visser]

On Apr 29, 2009, at 12:18 AM, Luke Palmer wrote:

> On Tue, Apr 28, 2009 at 4:09 PM, Vasili I. Galchin <vigalchin at gmail.com 
> > wrote:
> Hello,
>
>      Is there anyway when defining a dat type record struct to  
> indicate default values for some of the  fields?
>
> The usual pattern is to use a default record, and specialize it:
>
> data Foo = Foo { bar :: Int, baz :: Int, quux :: Int }
>
> fooDefault = Foo { bar = 1, baz = 2, quux = 3 }
>
> newRecord = fooDefault { quux = 42 }
>
> If you only want some of the fields to be defaulted, you can make  
> them undefined in the default record... unfortunately this won't be  
> checked by the compiler.
>
> Luke


Additionally, you can choose to not export the constructor Foo and to  
only export your fooDefault quasi-constructor and your record selectors.