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+---
+CIP: 92
+Title: First-class errors in Plutus
+Category: Plutus
+Status: Proposed
+Authors:
+    - Las Safin <me@las.rs>
+Implementors:
+    - Las Safin <me@las.rs>
+Discussions:
+    - https://github.com/cardano-foundation/cips/pulls/?
+Created: 2023-03-01
+License: CC-BY-4.0
+---
+
+## Abstract
+
+Plutus is a strictly evaluated language, where users heavily rely on **impure** errors
+to cause transaction failure to rule out invalid cases.
+
+We propose three changes:
+- Make `error` a first-class value you can pass around. Using it does not inherently
+  cause an error.
+- Change condition for script success from not erring to returning `True`.
+- Erase type abstractions and type applications entirely when going from
+  TPLC to UPLC rather than turning them into `delay` and `force`.
+
+UPLC is now **truly pure** wrt. transaction validation.
+This enables another use-case: We can **optimise** it as if it
+were entirely pure. We can turn `const () x` into `()` for any `x`!
+We can turn `expensive1 || expensive2` into `if expensive1 then True else expensive2`,
+and in general do any transformations that we'd do on a pure language without
+regard for side-effects.
+
+Issues fixed / PRs closed by this CIP:
+- https://github.com/input-output-hk/plutus/issues/4826
+- https://github.com/input-output-hk/plutus/issues/4114
+- https://github.com/input-output-hk/plutus/pull/4118
+- https://github.com/cardano-foundation/CIPs/pull/459
+
+## Motivation
+
+Users write code in the style of `if cond then error else False`, which is unidiomatic and also problematic,
+since the seemingly equivalent `cond && error` will **always err**.
+This is not a good situation, and issues have been opened about it before,
+notably https://github.com/input-output-hk/plutus/issues/4114 (over a year ago!).
+The issue was righly not deemed a "vulnerability" or "bug", yet it is quite clearly a misfeature.
+
+The current design in fact **encourages impurity**, as the return value of scripts is ignored entirely;
+only the effects matter. This is entirely antithetical to the design of Plutus.
+With the changes in this CIP, users will naturally return `False` in cases that are
+disallowed explicitly, `True` when it's allowed explicitly, and all other cases are
+disallowed "implicitly" as they're also not `True`.
+
+There is also the issue of efficiency: `delay` and `force`s have added more of an
+overhead than initially assumed, and a common trick used by alternative toolchains is to
+avoid emitting these. PlutusTx still emits these as it targets PIR, the erasure of which into UPLC
+adds this overhead.
+However, removing this from the erasure itself doesn't just help PlutusTx get closer to the alternative
+toolchains, but also means that all built-ins will no longer need to be forced before application.
+
+Making Plutus lazy would also solve the problems, yet this has not been done as a strict language
+is easier to reason about wrt. budgetting. Instead, we turn to making errors first-class values.
+
+As noted in the abstract, we can now also do optimisations on the UPLC that were impossible
+before, as the effects mattered. This notably meant we previously couldn't regard code which result
+wasn't used as dead code. With this change, it's possible to entirely elide such (now) dead code.
+
+One minor effect of this CIP is that evaluation strategy is less important, meaning that you
+could theoretically take a script in this new language and evaluate it lazily, without breaking
+anything.
+
+## Specification
+
+The changes below would need to take effect in a new ledger era and Plutus version.
+It is not yet clear which, as that depends on when it gets implemented.
+
+### Plutus Core
+
+To solve this problem at its root, this CIP proposes making errors **first-class values**.
+Errors are no longer a special type of term that "infect" their environment, but can instead
+be passed around freely. Using an error incorrectly is of course still an error, hence it still
+bubbles up in that respect.
+
+We remove the following small-step reduction rule:
+```
+f {(error)} -> (error)
+```
+from [Plutus Core Specification](https://github.com/input-output-hk/plutus/blob/72d82f9d04cbef144727fad1f86ce43476e4cc63/doc/plutus-core-spec/untyped-reduction.tex#L133).
+
+#### Built-ins
+
+Errors passed to a built-in only cause an error if the built-in attempts
+to inspect it, i.e. morally pattern-match on it in the Haskell definition.
+
+The matter of costing is a bit more complicated:
+Arguments are still always in WHNF, and hence we still derive the cost from their
+size. We can regard an error as a member of every type, and assign it a size in every type.
+The size can be any constant, as it would be if it were explicitly added as a new
+constructor to every built-in type.
+If it is used, evaluation of the built-in halts immediately.
+If it isn't used, it doesn't use any compute.
+Whether it should take from the budget if not used is up to the implementors.
+
+### Ledger
+
+[`runPLCScript`](https://github.com/input-output-hk/cardano-ledger/blob/f58187e1f2ebb7bbd9f5c3072cab5b1b6568c93f/eras/alonzo/formal-spec/utxo.tex#L213) from the ledger will need a new version that checks
+that the result is `True`, not that it didn't fail. This change can be done
+transparently in the Plutus library by adding another version of evaluateScriptRestricting
+with an equivalent type but for a new version of Plutus.
+
+## Rationale
+
+Plutus isn't made lazy by this change, errors are instead not immediately propagated up.
+Errors are less special-cased now, as evident by the removal of a rule, not addition.
+
+One (somewhat obvious) issue is that `error` isn't the only source of divergence.
+As @effectfully pointed out, the following is still problematic:
+```haskell
+let factorial n = ifThenElse (n == 0) 1 (factorial (n - 1))
+in factorial 2
+```
+The arguments to `ifThenElse` are evaluated first, leading to infinite recursion.
+Fixing this would require making Plutus to some extent lazy, which is a much larger change.
+
+Is this change still worth it in light of this?
+Even _purely_ for the sake of making scripts more functional, it would still be worth it.
+Very few people will make their own `loopforever` function to replicate the old `error` to write impure scripts.
+It is a bonus that we in addition 1) simplify the language 2) remove a foot-gun for new Plutusers/Plutors/Plutonites
+3) make Plutus scripts more efficient by removing `delay` and `force` from the erasure.
+
+It is not immediately obvious that the third step is possible, as we still have the above issue.
+The common example (from [Well-Typed](https://well-typed.com/blog/2022/08/plutus-cores/)) is:
+```haskell
+let* !boom = λ {a} -> ERROR
+     !id   = λ {a} (x :: a) -> x
+in λ (b :: Bool) -> ifThenElse {∀ a. a -> a} b boom id
+```
+Indeed, this example works _without_ adding delays to type abstractions,
+as the untyped form
+```haskell
+f = λ b -> ifThenElse b ERROR (λ x -> x)
+```
+is precisely the example we seek to make work.
+`f True ()` will fail as expected,
+while `f False ()` will return `()` as expected.
+
+### Soundness of optimisations
+
+We note that you can soundly optimise `(\x -> M) N` into `M` if `x` is not mentioned in `M`,
+or similar occurrences where the result of `N` isn't needed.
+Such an optimisation will change the behaviour of the script necessarily,
+as any traces in `N` will no longer happen, yet, this is not important in practice
+as traces are merely a debugging tool. The budget used will decrease,
+but people do not rely on budget exhaustion to signal invalidity.
+
+Transformations like `M || N` into `force (ifThenElse M (delay True) (delay N))` follow a similar principle:
+traces that happened before might no longer happen.
+The used budget will increase if M is `False`, yet this is likely
+still an acceptable transformation for most people.
+
+### Backward compatibility
+
+This CIP has the issue of old scripts not being portable to the new version trivially.
+You need to make sure your script isn't making use of any impure assumptions.
+
+## Path to Active
+
+### Acceptance Criteria
+
+It would be needed to implement the necessary changes in `cardano-ledger` and `plutus`.
+
+### Implementation Plan
+
+#### Implementation of built-ins
+
+This section does not have to be followed and is merely initial speculation
+on how to implement this feature.
+
+The implementation of built-in functions in the Plutus library must
+change to support built-ins that don't inspect all arguments, notably `ifThenElse`.
+The naive way would be to translate errors to `undefined` and catch that impurely, yet this is hardly satisfying.
+
+Built-ins defined by `makeBuiltinMeaning` would, for a Plutus function of type `a0 -> a1 -> ... -> an`,
+be defined by a Haskell-function `forall m. m a0 -> m a1 -> ... m an` instead.
+If you don't bind an argument, it is not used, hence evaluation will still succeed
+regardless of whether it is an error.
+If it is bound, but is an error, then it will return an error as usual.
+
+To reduce the changes needed, you could instead of changing `makeBuiltinMeaning` provide
+an alternative `makeLazyBuiltinMeaning` with the above functionality, changing the original
+function to instead bind all arguments immediately, as most built-ins would do anyway.
+
+With this, the implementation of `ifThenElse` would be
+```haskell
+toBuiltinMeaning DefaultFunVX IfThenElse =
+  makeLazyBuiltinMeaning
+    (\b x y -> b >>= \case True -> x ; False -> y)
+    (runCostingFunThreeArguments . paramIfThenElse)
+```
+
+## Copyright
+
+[CC-BY-4.0]: https://creativecommons.org/licenses/by/4.0/legalcode
+[Apache-2.0]: http://www.apache.org/licenses/LICENSE-2.0