Re: [Haskell-cafe] Modifying a product type generically

2 Jan 2015

      The absent case corresponds to a multi-constructor type. You need to
decide what to do in that case.

You can decide not to allow it, in which case you can enforce it by saying

  (xss ~ '[x], yss ~ '[a ': x])

(essentially replacing xs with '[]).

Or perhaps you do want to handle those cases; then it depends on how
exactly you want to map multiple constructors.

Roman

On 02/01/15 06:02, Michael Orlitzky wrote:
...
I've been playing with generic conversions between simple product types.
For example, converting (Bar Int Int) back and forth between (Int,Int). See,
https://www.haskell.org/pipermail/haskell-cafe/2014-December/117489.html
These save me a *lot* of boilerplate. But naturally, I've gotten greedy.
Another transformation I need to make takes one data type, and sticks a
new field on the beginning of it. So I'd like to be able to take a,
Foo Int Int
and construct a,
BigFoo String Int Int
with as little code as possible. In my real problem, Foo may have 30
fields, and BigFoo 31 -- if you know of a better way, stop me now.
And, I actually managed to do it using generics-sop:
...
prepend "Hello" (Foo 1 2) :: BigFoo
  BigFoo "Hello" 1 2
But, I had to do something dirty; I used an incomplete pattern match. I
don't know enough about what's going on here to handle the other case
for prepend_sop. I basically guessed the existing type signatures
because it wouldn't work without them.
I would like to either,
a) Get prepend_sop working for sums of products (the missing case),
     not just a single product (the one I've got working)
b) Restrict prepend_sop to the case that I've got working via the
     type system, somehow
In other words, I don't want it to be crashy, but I'm stuck. Here's the
complete working code for the example above.
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
module Prepend
where
import qualified GHC.Generics as GHC
import Generics.SOP
data Foo = Foo Int Int
  deriving (Show, GHC.Generic)
instance Generic Foo
-- A "copy" of Foo, with a String in front of it.
data BigFoo = BigFoo String Int Int
  deriving (Show, GHC.Generic)
instance Generic BigFoo
prepend_sop :: (xss ~ (x ': xs), (yss ~ ((a ': x) ': xs)))
        => a
        -> SOP I xss
        -> SOP I yss
prepend_sop z (SOP (Z rest)) = SOP $ Z ((I z) :* rest)
--prepend_sop z (SOP (S rest)) = ???
prepend :: (Generic a, Generic c, Code a ~ (x ': xs),
            Code c ~ ((b ': x) ': xs))
        => b
        -> a
        -> c
prepend z = to . (prepend_sop z) . from
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Re: [Haskell-cafe] Modifying a product type generically

Roman Cheplyaka