[Haskell-cafe] Alternatives to convoluted record syntax

Henning Thielemann lemming at henning-thielemann.de
Thu Jul 3 06:13:26 EDT 2008

On Thu, 3 Jul 2008, Dougal Stanton wrote:

> Here's a snippet from the parser for one option (others omitted for clarity):
>> options :: [OptDescr (Opts -> Opts)]
>> options =
>>     [ Option "b" ["bus"] (ReqArg busNum "NUM") "Bus number"
>>     , ...
>>     ]
>>   where busNum n os = let b = (query os) { queryBusNumber = Just n }
>>                       in if isBusId n then os { query = b } else os
> Variations on that ugliness are repeated four times for other fields.
> Is there an alternative way to change the value of nested fields?

For access to nested record fields I implemented the record-access 
package. Unfortunately the field accessors must still be written manually.


infix2 :: ((Char, Int), String)
infix2 =

(second^=10) replaces the second member of pair with a value.
(^:) applies that change to the outer record (again a pair).
($%) applies the modifier (first^:second^=10) to a concrete pair.

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