{-# OPTIONS_GHC -Wno-redundant-constraints #-}
module Plutarch.Docs.PBuiltinList (validBuiltinList, listOfBs, matchOnList, matchOnList') where 
import Plutarch.Prelude

PBuiltinList

You’ll be using builtin lists quite a lot in Plutarch. PBuiltinList has a PListLike instance, giving you access to all the goodies from there! However, PBuiltinList can only contain builtin types. In particular, it cannot contain Plutarch functions (which also implies it cannot contain scott-encoded datatypes).

You can express the constraint of “only builtin types” using PLift, exported from Plutarch.Builtin-

validBuiltinList :: forall a s. PLift a => Term s (PBuiltinList a)
validBuiltinList = undefined

As mentioned before, PBuiltinList gets access to all the PListLike utilities. Other than that, PLift a => PBuiltinList a also has a PlutusType instance. You can construct a PBuiltinList using pcon (but you should prefer using pcons from PListLike):

listOfBs :: forall s. Term s (PBuiltinList (PAsData PByteString))
listOfBs = pcon $ PCons (pdata $ phexByteStr "fe") $ pcon PNil

would yield a PBuiltinList (PAsData PByteString) with one element - 0xfe. Of course, you could have done that with pcons # pdata (phexByteStr "fe") # pnil instead!

You can also use pmatch to match on a list:

matchOnList :: forall s. Term s PString
matchOnList = pmatch (pcon $ PCons (phexByteStr "fe") $ pcon PNil) $ \case
  PNil -> "hey hey there's nothing here!"
  PCons _ _ -> "oooo fancy!"

But you should prefer pelimList from PListLike instead:

matchOnList' :: forall s. Term s PString
matchOnList' = pelimList (\_ _ -> "oooo fancy") "hey hey there's nothing here!" $ pcon $ PCons (phexByteStr "fe") $ pcon PNil

The first argument is a function that is invoked for the PCons case, with the head and tail of the list as arguments.

The second argument is the value to return when the list is empty. It’s only evaluated if the list is empty.

The final argument is, of course, the list itself.

Aside: Interested in the lower level details of PBuiltinList (i.e. Plutus Core builtin lists)? You can find all you need to know about it at Plutonomicon.