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Remove the deriving-aeson dependency #7871

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c6ad540
Add tests for cost model JSON encoding stability
Unisay Jul 28, 2026
63512e4
Remove the deriving-aeson dependency
Unisay Jul 28, 2026
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7 changes: 7 additions & 0 deletions .../changelog.d/20260728_100010_yuriy.lazaryev_issue_7870_remove_deriving_aeson.md
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@@ -0,0 +1,7 @@
### Removed

- `LowerInitialCharacter` is no longer exported from `PlutusCore.Evaluation.Machine.ExBudget`; it existed solely to support `deriving-aeson`, which `plutus-core` no longer depends on.

### Changed

- Replaced the `deriving-aeson`-based JSON instances of the cost model types with plain `aeson` generic instances. The JSON format is unchanged.
2 changes: 1 addition & 1 deletion plutus-core/cost-model/CostModelGeneration.md
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Expand Up @@ -292,7 +292,7 @@ our case, the costing function is given by the
type. The type prefix `ModelTwoArguments` is removed from the
constructor name and the remaining `MinSize` is converted to
`min_size` by the Aeson library's
[`CamelToSnake`](https://hackage.haskell.org/package/deriving-aeson-0.2.8/docs/Deriving-Aeson.html#t:CamelToSnake)
[`camelTo2`](https://hackage.haskell.org/package/aeson/docs/Data-Aeson.html#v:camelTo2)

@kwxm kwxm Aug 1, 2026

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I was a bit worried that these might give different results, but if you look at the implementation in deriving-aeson it has

data CamelTo (separator :: Symbol)
...
type CamelToSnake = CamelTo  "_"
...
instance (KnownSymbol separator, NonEmptyString separator) => StringModifier (CamelTo separator) where
  getStringModifier = camelTo2 char
    where
      char = case symbolVal (Proxy @separator) of
        c : _ -> c
        _ -> error "Impossible"

so the previous code was secretly using camelTo2 anyway.

It does look as if you have to be a bit careful about what conversion functions like this do in general. For example we have a constructor called LinearInMaxYZ and there's a question of what happens if you have a sequence of multiple uppercase letters. In this case camelTo2 '_' gives linear_in_max_yz, but linear_in_max_y_z might be justifiable as well (and it looks as if there are functions in other libraries that would do that).

transformation. Similarly, the names of the
`modelMinSizeIntercept` and `modelMinSizeSlope` fields in the
`ModelMinSize` type are converted to `slope` and `intercept`. In
Expand Down
5 changes: 4 additions & 1 deletion plutus-core/plutus-core.cabal
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Expand Up @@ -327,7 +327,6 @@ library
, data-default-class
, deepseq
, dependent-sum >=0.7.1.0
, deriving-aeson >=0.2.3
, deriving-compat
, dlist
, exceptions
Expand Down Expand Up @@ -382,6 +381,7 @@ test-suite plutus-core-test
CBOR.DataStability
Check.Spec
CostModelInterface.Spec
CostModelJSON.Spec
CostModelSafety.Spec
Evaluation.Machines
Evaluation.Spec
Expand All @@ -398,6 +398,9 @@ test-suite plutus-core-test
default-language: Haskell2010
build-depends:
, aeson
, aeson-diff
, aeson-pretty
, barbies
, base >=4.9 && <5
, base16-bytestring ^>=1.0
, bytestring
Expand Down
43 changes: 22 additions & 21 deletions plutus-core/plutus-core/src/PlutusCore/Evaluation/Machine/BuiltinCostModel.hs
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@@ -1,6 +1,5 @@
-- editorconfig-checker-disable-file
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE StrictData #-}
Expand Down Expand Up @@ -57,13 +56,12 @@ import PlutusPrelude hiding (toList)

import PlutusCore.Evaluation.Machine.CostingFun.Core
import PlutusCore.Evaluation.Machine.CostingFun.JSON ()
import PlutusCore.Evaluation.Machine.ExBudget

import Barbies

@kwxm kwxm Aug 1, 2026

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Maybe qualify this so we can see what we're using?

I tried import Barbies (ConstraintsB, TraversableB, FunctorB) but that mysteriously gave me several errors like this:

plutus-core/src/PlutusCore/Evaluation/Machine/BuiltinCostModel.hs:202:46: error: [GHC-18872]
    • Couldn't match representation of type: GHC.Generics.K1
                                               GHC.Generics.R (f ModelFourArguments)
                               with that of: barbies-2.1.1.0:Data.Generics.GenericN.Rec
                                               (barbies-2.1.1.0:Data.Generics.GenericN.Param
                                                  0 f ModelFourArguments)
                                               (f ModelFourArguments)
        arising from the 'deriving' clause of a data type declaration
        The data constructor ‘GHC.Generics.K1’
          of newtype ‘GHC.Generics.K1’ is not in scope
    • When deriving the instance for (ConstraintsB
                                        BuiltinCostModelBase)
    |
202 |   deriving anyclass (FunctorB, TraversableB, ConstraintsB)
    |                                              ^^^^^^^^^^^^

Not sure what's going on there.

import Data.Aeson
import Data.Kind qualified as Kind
import Data.List (stripPrefix)
import Data.Monoid
import Deriving.Aeson
import Language.Haskell.TH.Syntax hiding (Name, newName)

type BuiltinCostModel = BuiltinCostModelBase CostingFun
Expand Down Expand Up @@ -203,18 +201,21 @@ data BuiltinCostModelBase f
deriving stock (Generic)
deriving anyclass (FunctorB, TraversableB, ConstraintsB)

deriving via
CustomJSON
'[FieldLabelModifier (StripPrefix "param", LowerInitialCharacter)]
(BuiltinCostModelBase CostingFun)
instance
ToJSON (BuiltinCostModelBase CostingFun)
deriving via
CustomJSON
'[FieldLabelModifier (StripPrefix "param", LowerInitialCharacter)]
(BuiltinCostModelBase CostingFun)
instance
FromJSON (BuiltinCostModelBase CostingFun)
{-| JSON options for 'BuiltinCostModelBase': drop the @param@ prefix and lower the initial
character, so that the names of the JSON fields are exactly the same as the names of the
builtins. -}
builtinCostModelOptions :: Options
builtinCostModelOptions =
defaultOptions {fieldLabelModifier = lowerInitialChar . dropPrefix "param"}
where
dropPrefix prefix s = fromMaybe s (stripPrefix prefix s)

instance ToJSON (BuiltinCostModelBase CostingFun) where
toJSON = genericToJSON builtinCostModelOptions
toEncoding = genericToEncoding builtinCostModelOptions

instance FromJSON (BuiltinCostModelBase CostingFun) where
parseJSON = genericParseJSON builtinCostModelOptions

{-| Same as 'CostingFun' but maybe missing.
We could use 'Compose Maybe CostinFun' instead but we would then need an orphan ToJSON instance. -}
Expand All @@ -223,12 +224,12 @@ newtype MCostingFun a = MCostingFun (Maybe (CostingFun a))
deriving (Semigroup, Monoid) via (Alt Maybe (CostingFun a)) -- for mempty == MCostingFun Nothing

-- Omit generating JSON for any costing functions that have not been set (are missing).
deriving via
CustomJSON
'[OmitNothingFields, FieldLabelModifier (StripPrefix "param", LowerInitialCharacter)]
(BuiltinCostModelBase MCostingFun)
instance
ToJSON (BuiltinCostModelBase MCostingFun)
instance ToJSON (BuiltinCostModelBase MCostingFun) where
toJSON = genericToJSON mBuiltinCostModelOptions
toEncoding = genericToEncoding mBuiltinCostModelOptions

mBuiltinCostModelOptions :: Options
mBuiltinCostModelOptions = builtinCostModelOptions {omitNothingFields = True}

-- Needed to help derive various instances for BuiltinCostModelBase
type AllArgumentModels (constraint :: Kind.Type -> Kind.Constraint) f =
Expand Down
2 changes: 1 addition & 1 deletion plutus-core/plutus-core/src/PlutusCore/Evaluation/Machine/CostingFun/Core.hs
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Expand Up @@ -59,8 +59,8 @@ import PlutusCore.Evaluation.Machine.ExMemoryUsage
import Control.DeepSeq
import Data.Default.Class
import Data.Hashable
import Deriving.Aeson
import GHC.Exts
import GHC.Generics (Generic)
import Language.Haskell.TH.Syntax hiding
( Name
, newName
Expand Down
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