Generic structural diffs

generic-diff lets us pinpoint exactly where two values differ, which can be very useful, for example for debugging failing tests. This functionality is provided by the Diff typeclass, for which instances can be derived automatically using Generic from generics-sop.

For detailed information, see the Hackage docs.

Example

{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DeriveAnyClass #-}

import Generics.Diff
import Generics.Diff.Render

import qualified GHC.Generics as G
import qualified Generics.SOP as SOP

data BinOp = Plus | Minus
  deriving stock (Show, G.Generic)
  deriving anyclass (SOP.Generic, SOP.HasDatatypeInfo, Diff)

data Expr
  = Atom Int
  | Bin {left :: Expr, op :: BinOp, right :: Expr}
  deriving stock (Show, G.Generic)
  deriving anyclass (SOP.Generic, SOP.HasDatatypeInfo, Diff)

expr1, expr2 :: Expr
expr1 = Bin (Atom 1) Plus (Bin (Atom 1) Plus (Atom 1))
expr2 = Bin (Atom 1) Plus (Bin (Atom 1) Plus (Atom 2))

ghci> printDiffResult $ diff expr1 expr2
In field right:
  Both values use constructor Bin but fields don't match
  In field right:
    Both values use constructor Atom but fields don't match
    In field 0 (0-indexed):
      Not equal