[Haskell-cafe] default values in a record structure

[Luke Palmer]

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
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