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imports 

{-# OPTIONS_GHC -Wno-unused-imports #-}
{-# LANGUAGE TemplateHaskell #-}
module Plutarch.Docs.FFI () where 
import Plutarch.Prelude
import Plutarch.FFI

import PlutusTx qualified
import PlutusTx.Builtins qualified as PlutusTx
import PlutusTx.Builtins.Internal qualified as PlutusTx

import Generics.SOP qualified as SOP
import Data.Default (def)

Interoperability with PlutusTx

Note: This module needs the PlutusTx plugin. As it does not yet work under ghc92+, some of this documentation is not part of the CI, the documentation is up to date as of Plutarch 1.3.

If you already have a codebase built using PlutusTx, you can choose to re-write only its critical parts in Plutarch and to call them from PlutusTx. The function to use is Plutarch.FFI.foreignExport:

doubleInPlutarch :: ClosedTerm (PInteger :--> PInteger)
doubleInPlutarch = plam (2 *)

doubleExported :: PlutusTx.CompiledCode (Integer -> Integer)
doubleExported = foreignExport def doubleInPlutarch

doubleUseInPlutusTx :: PlutusTx.CompiledCode Integer
doubleUseInPlutusTx = doubleExported `PlutusTx.applyCode` PlutusTx.liftCode 21

Alternatively, you may go in the opposite direction and call an existing PlutusTx function from Plutarch using Plutarch.FFI.foreignImport:

doubleInPlutusTx :: PlutusTx.CompiledCode (Integer -> Integer)
doubleInPlutusTx = $$(PlutusTx.compile [||(2 *) :: Integer -> Integer||])

doubleImported :: Term s (PInteger :--> PInteger)
doubleImported = foreignImport doubleInPlutusTx

doubleUseInPlutarch :: Term s PInteger
doubleUseInPlutarch = doubleImported # 21

Note how Plutarch type PInteger :--> PInteger corresponds to Haskell function type Integer -> Integer. If the types didn't correspond, the foreignExport and foreignImport applications wouldn't compile. The following table shows the correspondence between the two universes of types:

Plutarch Haskell
pa :--> pb a -> b
PTxList pa [a]
PTxMaybe pa Maybe a
PInteger Integer
PBool BuiltinBool
PString BuiltinString
PByteString BuiltinByteString
PBuiltinData Data
PUnit BuiltinUnit
PDelayed pa Delayed a

User-defined types

When it comes to user-defined types, you have a choice of passing their values encoded as Data or directly. In the latter case, you'll have to declare your type twice with two kinds: as a Haskell Type and as a Plutarch PType. Furthermore, both types must be instances of SOP.Generic, as in this example:

data SampleRecord = SampleRecord
  { sampleBool :: PlutusTx.BuiltinBool
  , sampleInt :: PlutusTx.Integer
  , sampleString :: PlutusTx.BuiltinString
  }
  deriving stock (Generic)
  deriving anyclass (SOP.Generic)

data PSampleRecord (s :: S) = PSampleRecord
  { psampleBool :: Term s PBool
  , psampleInt :: Term s PInteger
  , psampleString :: Term s PString
  }
  deriving stock Generic
  deriving anyclass (SOP.Generic, PlutusType)
instance DerivePlutusType PSampleRecord where type DPTStrat _ = PlutusTypeScott

With these two declarations in place, the preceding table can gain another row:

Plutarch Haskell
PDelayed PSampleRecord SampleRecord

The reason for PDelayed above is a slight difference in Scott encodings of data types between Plutarch and PlutusTx. It means you'll need to apply pdelay to a PSampleRecord value before passing it through FFI to Haskell, and pforce after passing it in the opposite direction.

This technique can be used for most data types, but it doesn't cover recursive types (such as lists) nor data types with nullary constructors (such as Maybe). To interface with these two common Haskell types, use PTxMaybe and PTxList types from Plutarch.FFI. The module also exports the means to convert between these special purpose types and the regular Plutarch PMaybe and PList.