How to make a record assignment parametric in Haskell?

Question

Suppose we have a datatype like this:

data NodeProperties = 
    NodeProperties
    { 
        p1 :: PropertyInfo Bool,
        p2 :: PropertyInfo [String],
        p3 :: PropertyInfo [String],
        p4 :: PropertyInfo Bool,
        p5 :: PropertyInfo [String]
    } 
    deriving (Show, Data, Typeable)

and we have a modify function that has this kind of clauses:

| p1Prefix /= Nothing =
    let 
        value = readBoolean p1Prefix
        newProperties = 
            properties
            { 
                p1 = (p1 properties){value = Just value}
            }
    in 
        modifyPropertiesNode pragmas newProperties
| p2Prefix /= Nothing =
    let 
        value = readStringList p2Prefix
        newProperties = 
            properties
            { 
                p2 = (p2 properties){value = Just value}
            }
    in 
        modifyPropertiesNode pragmas newProperties

In my opinion, it would be better to have code above generalized to something like the code below.

| p1Prefix /= Nothing =
    updateProperty p1 readBoolean p1Prefix pragmas
| p2Prefix /= Nothing =
    updateProperty p2 readStringList p2Prefix pragmas

In order to do something like this I would need to use variables at the left hand side of a record assignment, and it seems that Haskell does not like the idea. Is there any alternative to achieve this?

You want [lenses](https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/a-little-lens-starter-tutorial). — n. m. could be an AI, Dec 01 '15 at 10:35
@n.m.Thanks for your proposal. It can be a solution, but I would like to know if there is an alternative way to do this using the Haskell syntax or standard libraries. — Salvador Tamarit, Dec 01 '15 at 11:16
No, there is not, but there are a variety of lens implementations. — Rein Henrichs, Dec 01 '15 at 12:11
Don't think so. That's why lenses where invented in the first place. Not because doing it with standard libraries would be too easy. — n. m. could be an AI, Dec 01 '15 at 13:28
@ReinHenrichs you are right. Using Lenses is a very nice way to achieve my goal. On the other hand, it seems that the compilation time has grown considerably after importing Lenses library and adding TemplateHaskell directive. — Salvador Tamarit, Dec 01 '15 at 16:50
@SalvadorTamarit You may be interested in a lighter weight lens library, but after lens itself is compiled the compilation times for your project shouldn't be that much higher. — Rein Henrichs, Dec 01 '15 at 19:40

How to make a record assignment parametric in Haskell?

0 Answers0