IntersectMBO/plutus

Investigate if lambdas not annotated with types fail to compile in Plutus Tx

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#6,085 opened on May 22, 2024

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Good first issueInternalPlinthUser experiencestatus: triaged

Description

James Browning reported this issue.

In an earlier plutus version I was able to compile (\_ _ -> ()) or (\_ _ _ -> ()) but this now produces an error

GHC Core to PLC plugin: Error: Unsupported feature: Kind: forall k. k
Context: Compiling kind: forall k. k
Context: Compiling type: GHC.Types.Any
Context: Compiling expr: \ _ [Occ=Dead] _ [Occ=Dead] -> GHC.Tuple.()

Using a BuiltinData -> BuiltinData -> () function still works. Just wondered why this might be and if we care enough

For example, I used to use

alwaysSucceedPolicy :: MintingPolicy
alwaysSucceedPolicy = mkMintingPolicyScript $$(PlutusTx.compile [|| (\_ _ -> ()) ||])

but now I must do like

mkAlwaysSucceedPolicy :: BuiltinData -> BuiltinData -> ()
mkAlwaysSucceedPolicy _datum _sc = ()

alwaysSucceedPolicy :: SerialisedScript
alwaysSucceedPolicy = serialiseCompiledCode $$(PlutusTx.compile [|| mkAlwaysSucceedPolicy ||])

I think it wouldn’t be surprising if we couldn’t compile a naked \_ _ -> () however mkMintingPolicyScript does constrain the type of each argument bound by a lambda to BuiltinData, so in theory all types are there and GHC should be able to see them. In practice it may of course all be much more complicated than that.

Would be great to make it work, but perhaps not a huge deal if we can’t.

Contributor guide

Tech stack
None
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
needs investigation
Prerequisites
HaskellPlutus TxGHC Core
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate why lambdas without type annotations fail to compile in Plutus Tx. The error originates from GHC Core to PLC plugin: 'Unsupported feature: Kind: forall k. k'. Look at the compilation pipeline for `mkMintingPolicyScript` which constrains types to BuiltinData. Check if GHC can infer types properly. Examine the GHC Core representation of `\ > ()` and why it generates a forall kind. Possibly related to handling of kind polymorphism in Plutus Tx. See comments from James Browning and maintainers. Relevant files: PlutusTx/Compile.hs and GHC Core to PLC conversion code.