[Haskell] Proposal: Allow "\=" for field update in record update syntax

[Keean Schupke]

Its a pity template haskell cannot define infix operators, but you could 
use TH like:

    $update rec field fn

which would expand to:

    rec { field = fn (rec field)  }

Keean. 

S. Alexander Jacobson wrote:

> I don't know what it takes to get this sort of change into 
> circulation, but I assume it starts with a clear and specific 
> description of the problem, exploration of alternatives, and a 
> specific proposed solution.  So here goes:
>
> ----------
> Proposal: Allow "\=" for field update in record update syntax
>
> Specifically, intepret
>
>    rec {field\=fn}
>
> as
>
>    rec {field = fn (field rec)}
>
> Jusitification:
>
> It is extremely common when updating records to be updating a field 
> value based on its prior value rather than simply assigning a new 
> value from nothing.  Unforunately the syntax supports the later 
> clearly but makes the former awkward to read, understand, and debug:
>
>    myFun db myType1 =
>       db { tMyType1 = insert myType1 $ tMyType1 db
>            nextId = 1 + (nextId db)
>          }
>
> One solution to this problem that does not involve changing syntax is 
> to generate field update functions for every field of a type e.g.:
>
>    update_tMyType1 f s = s {tMyType1= f $ myType1 s}
>    update_nextId f s = s {nextId = f $ nextId s}
>
> Once you have those functions, myFun looks much better:
>
>    myFun db myType = update_nextId (+1) $ update_tMyType1 f db
>      where f= insert myType1
>
> But, generating the update_ functions is not programming; its just 
> manual syntax generation.  And, even if you have template haskell to 
> do it,
>
>   * it is annoying to have to find/write the TH code to implement
>   * update_ is incredibly verbose
>   * shortening it risks proliferation of psuedo-syntax:
>      u_tMyType, u__tMyType, or utMyType?
>   * it adds clutter to have to call the TH for each record type, and
>   * it pollutes the name space/increases risk of name collision
>
> Rather than force the user to jump through these hoops, why not make 
> the syntax flow naturally from the existing record update syntax? It 
> makes sense to use \= for this task as it is the functional 
> generalization of C or Java's += or *= . Then myFun would have the 
> elegant and clear syntax:
>
>     myFun db myType1 =db {tMyType1\=insert myType1, nextId\=(+1)}
>
> -Alex-
>
>
> ______________________________________________________________
> S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com
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