CIP-0168? | More BuiltinValue Functions

CIP-0168/README.md

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+---
+CIP: 168
+Title: More `BuiltinValue` Functions
+Category: Plutus
+Status: Proposed
+Authors:
+    - fallen-icarus <[email protected]>
+Implementors: []
+Discussions: 
+    - https://github.com/cardano-foundation/CIPs/pull/1090
+Created: 2025-10-01
+License: CC-BY-4.0
+---
+
+## Abstract
+
+[CIP-0153][1] added a new `BuiltinValue` type to make working with on-chain values more efficient.
+However, it added only a few builtin functions for working with this new `BuiltinValue` type which
+limits its real world usability. Adding and maintaining builtin functions is costly, but the
+importance of validating values in the eUTxO model justifies a wider range of builtin functions for
+this purpose. With this in mind, this CIP proposes a few extra builtin functions to improve the
+usability of this new `BuiltinValue` type while still trying to minimize the overall maintenance
+burden.
+
+## Motivation: why is this CIP necessary?
+
+> UPLC is not a general-purpose programming language. It is a language specifically designed to
+> write validation logic to set constraints for the creation and transfer of Value and Data across
+> the Cardano blockchain. **Given that half of the entire purpose of this language is to express
+> constraints over the creation and transfer of** `Value`, why is Value treated as a standard
+> library concept rather than a first-class language primitive?
+
+This was the motivation for [CIP-0153][1]. Given the overwhelming importance for validating `Value`,
+it makes sense for `BuiltinValue` to have a reasonably disproportionate number of builtin operations
+as compared to `BuiltinList` or `BuiltinArray`. The current builtins added in CIP-0153 do not cover
+many of the use cases for `Value`. As a reference, see the Aiken stdlib's [functions on `Value`][2].
+Functions like `tokens`, `negate`, and `policies` are not possible to implement using the CIP-0153
+builtin functions which means they will not benefit from the new `BuiltinValue` type. Plutus needs
+enough builtins to cover the main operations on `BuiltinValue`, either through generalizable
+builtins or dedicated ones.
+
+## Specification
+
+This CIP proposes adding the following new builtins functions:
+
+```haskell
+type BuiltinCurrencySymbol = BuiltinByteString
+
+-- | Negate all values in a `BuiltinValue`.
+negate :: BuiltinValue -> BuiltinValue
+
+-- | Intersection of two `BuiltinValue`s. Returns data in the first value for the (policy ids, token
+-- names) existing in both values.
+intersection :: BuiltinValue -> BuiltinValue -> BuiltinValue
+
+-- | Returns all policy ids found in the value.
+policies :: BuiltinValue -> List [BuiltinCurrencySymbol]
+
+-- | Return all tokens and their quantities for a given policy id. It returns a `BuiltinValue` so
+-- that the result can make use of the `lookupCoin` builtin added in CIP-0153. It can always be
+-- converted to a `List` for pattern-matching using the `valueData` builtin added in CIP-0153.
+-- See the Rationale section for why `intersection` isn't used instead.
+lookupTokens :: BuiltinCurrencySymbol -> BuiltinValue -> BuiltinValue
+```
+
+## Rationale: how does this CIP achieve its goals?
+
+After this CIP, the total set of `BuiltinValue` functions will be:
+
+```haskell
+-- CIP-0153 functions
+insertCoin :: BuiltinCurrencySymbol -> BuiltinTokenName -> BuiltinInteger -> BuiltinValue -> BuiltinValue
+lookupCoin :: BuiltinValue -> BuiltinCurrencySymbol -> BuiltinTokenName -> BuiltinInteger
+valueContains :: BuiltinValue -> BuiltinValue -> Bool
+unionValue :: BuiltinValue -> BuiltinValue -> BuiltinValue
+valueData :: BuiltinValue -> BuiltinData
+unValueData :: BuiltinData -> BuiltinValue
+
+-- New functions
+negate :: BuiltinValue -> BuiltinValue
+intersection :: BuiltinValue -> BuiltinValue -> BuiltinValue
+policies :: BuiltinValue -> List [BuiltinCurrencySymbol]
+lookupTokens :: BuiltinCurrencySymbol -> BuiltinValue -> BuiltinValue
+```
+
+From these builtin functions, most of Aiken's stdlib `Value` functions can now make use of the
+improved efficiency of CIP-0153's `BuiltinValue`. The single `intersection` function enables all
+kinds of higher-level filtering functions using only a single builtin function. While `intersection`
+is not a trivial function, it is very similar to `unionValue` which has already been costed for
+CIP-0153.
+
+> [!NOTE] 
+> While `lookupTokens` could also be implemented using `intersection`, `lookupTokens` is just a
+> simple outer map value lookup. It should be a very simple builtin to cost and maintain so
+> accepting the performance hit from `intersection` instead likely isn't justified. For reference,
+> Aiken's `tokens` function is used many times in a single contract execution (once per tx output)
+> for protocols designed based on [CIP-89][9] which means the overhead from using `intersection`
+> would quickly add up.
+
+### Does this create a "Slippery Slope"?
+
+Aside from `intersection`, all other proposed builtins are simple operations commonly found on `Map`
+data structures. So the author does not believe this CIP sets a precedence for "builtin bloat".
+
+## Path to Active
+
+### Acceptance Criteria
+
+- [ ] The feature is implemented according to the implementation plan and merged into
+the master branch of the [plutus][3] repository.
+- [ ] [cardano-ledger][4] is updated to include new protocol parameters to control costing of
+the new builtins.
+- [ ] The feature is integrated into [cardano-node][5] and released as part of a hard fork.
+
+### Implementation Plan
+
+The implementation of this CIP should not proceed without an empirical assessment of the
+effectiveness of the new primitives, as per the following plan:
+
+1. Implement the new primitives according to the specification.
+2. Assign a preliminary cost to the new builtin functions. Consider similar operations and their
+   current costs.
+3. Create variants of the [existing benchmarks][6] and potentially add some more.
+4. Check that the builtin operations over `BuiltinValue` are indeed significantly faster.
+5. Check that `lookupTokens` is significantly faster and/or uses significantly less execution memory
+   than a comparable implementation using `intersection`.
+
+If the preliminary performance investigation was not successful, this CIP should be revised
+according to the findings of the experiment. Otherwise, the implementation can proceed:
+
+6. Determine the most appropriate costing functions for modelling the builtin's performance
+and assign costs accordingly.
+7. Add the new builtin type and functions to the appropriate sections in the [Plutus Core
+Specification][7].
+8. Formalize the new builtin type and functions in the [plutus-metatheory][8].
+9. The final version of the feature is ready to be merged into [plutus][3] and accepted by
+the Plutus Core team.
+
+## Copyright
+This CIP is licensed under [CC-BY-4.0](https://creativecommons.org/licenses/by/4.0/legalcode).
+
+[1]: https://github.com/cardano-foundation/CIPs/tree/master/CIP-0153/README.md "CIP-0153"
+[2]: https://aiken-lang.github.io/stdlib/cardano/assets.html "Aiken `Value` functions"
+[3]: https://github.com/IntersectMBO/plutus/ "plutus"
+[4]: https://github.com/IntersectMBO/cardano-ledger "cardano-ledger"
+[5]: https://github.com/IntersectMBO/cardano-node "cardano-node"
+[6]: https://github.com/IntersectMBO/plutus/tree/master/plutus-benchmark/script-contexts "script-context-benchmarks"
+[7]: https://plutus.cardano.intersectmbo.org/resources/plutus-core-spec.pdf "Formal Specification of the Plutus Core Language"
+[8]: https://github.com/IntersectMBO/plutus/tree/master/plutus-metatheory "plutus-metatheory"
+[9]: https://github.com/cardano-foundation/CIPs/blob/master/CIP-0089/README.md "Beacon Tokens CIP"