imports

{-# OPTIONS_GHC -Wno-redundant-constraints #-}
module Plutarch.Docs.PListLike (pfoo, pfoo', x, x') where 
import Plutarch.Prelude

`PListLike`

The PListLike typeclass bestows beautiful and familiar list utilities to its instances. Plutarch has two list types- PBuiltinList and PList. Both have PListLike instances! However, PBuiltinList can only contain builtin types. That means that, for example, it cannot contain Plutarch functions. The element type of PBuiltinList can be constrained using PLift a => PBuiltinList a.

As long as it’s a PLift a => PBuiltinList a or PList a - it has access to all the PListLike goodies, out of the box. It helps to look into some of these functions at Plutarch.List.

Along the way, you might be confronted by 2 big mean baddies …err, constraints:

PIsListLike list a

This just means that the type list a, is indeed a valid PListLike containing valid elements! Of course, all PList as are valid PListLike, but we have to think about PBuiltinList since it can only contain PLift a => a elements! So, in essence a function declared as:

pfoo :: PIsListLike list a => Term s (list a :--> list a)
pfoo = undefined

when specialized to PBuiltinList, can be simplified as:

pfoo' :: PLift a => Term s (PBuiltinList a :--> PBuiltinList a)
pfoo' = undefined

That’s all it is. Don’t be scared of it!

What about this one:

PElemConstraint list a

This one ensures that the element type a can indeed be contained within the list type - list. For PList, this constraint means nothing - it’s always true. For PBuiltinList, it can be simplified as PLift a. Easy!

Here’s two of my favorite PListLike utilities (not biased):

-- | Cons an element onto an existing list.
pcons :: PElemConstraint list a => Term s (a :--> list a :--> list a)

-- | The empty list
pnil :: PElemConstraint list a => Term s (list a)

What would life be without cons and nil?

Let’s build a PBuiltinList of PIntegers with that:

x :: Term s (PBuiltinList PInteger)
x = pcons # 1 #$ pcons # 2 #$ pcons # 3 # pnil

Wooo! Let’s not leave PList alone in the corner though:

x' :: Term s (PList PInteger)
x' = pcons # 1 #$ pcons # 2 #$ pcons # 3 # pnil

The code is the same, we just changed the type annotation. Cool!