[Haskell-cafe] Alternatives to convoluted record syntax

Misha Aizatulin avatar at hot.ee
Fri Jul 4 05:29:09 EDT 2008


>   busNum n
>     | (isBusId n) = $(modify 'query) ($(set 'queryBusNumber) (Just n))
>     | otherwise = id

    The solution I am using is creating for each record type @Rec@, and
each of its fields @fieldName :: T@ an updater

    updateFieldName :: (T -> T) -> Rec -> Rec

    This way you can write

busNum n
       | (isBusId n) = updateQuery $ updateQueryBusNumber $ const (Just n)
       | otherwise   = id

    The task of creating updaters can be automated using TH, this is what
the attached library does: all you need is to say

$(genUpdaters ''Opts)
$(genUpdaters ''Query)


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