imports

module Plutarch.Docs.Introduction (hf) where 
import Plutarch.Prelude

> Note: If you spot any mistakes/have any related questions that this guide lacks the answer to, please don't hesitate to raise an issue. The goal is to have high-quality documentation for Plutarch users!

Overview

Plutarch is an eDSL in Haskell for writing on-chain scripts for Cardano. With some caveats, Plutarch is a simply-typed lambda calculus (or STLC). Writing a script in Plutarch allows us to leverage the language features provided by Haskell while retaining the ability to compile to compact Untyped Plutus Core (or UPLC, which is an untyped lambda calculus).

When we talk about “Plutarch scripts,” we are referring to values of type Term (s :: S) (a :: PType). Term is a newtype wrapper around a more complex type, the details of which Plutarch end-users can ignore. A Term is a typed lambda term; it can be thought of as representing a computation that, if successfully evaluated, will return a value of type a.

The two type variables of the Term s a declaration have particular kinds:

s :: S is like the s of ST s a. It represents the computation context in a manner that mimics mutable state while providing a familiar functional interface. Sections 1 through 4 of [1] give an accessible introduction to how this works. s is never instantiated with a concrete value; it is merely a type-level way to ensure that computational contexts remain properly encapsulated (i.e., different state threads don’t interact). For more in-depth coverage of this and other eDSL design principles used in Plutarch, see [2].
a :: PType is short-hand for “Plutarch Type”. We prefix these types with a capital P, such as PInteger, PBool, and so forth. Tagging a Term with a PType indicates the type of the Term’s return value. Doing this allows us to bridge between the simple type system of Plutarch and the untyped UPLC.

Note that we should not think of a type of kind PType as carrying a value; it is a tag for a computation that may produce a value. For instance, the definition of PInteger is simply

data PInteger s

That is, there are no data constructors. If a value of type Term s PInteger successfully executes the computation within context s, the value computed will be an integer. We will never encounter values such as y :: PInteger; y = 3; they simply do not exist. While readers new to Plutarch may need some time to fit this into their mental model, it is a crucial distinction to keep in mind.

For brevity, we will say a “value of type Term s a will evaluate to (…)”. This phrase will carry two implicit notions: one, that Term s a represents a computation executed in the context s; two, evaluating Term s a is not guaranteed to succeed.

In brief, when writing Plutarch scripts, we have a few tasks:

A.) Defining Plutarch Types (or PTypes). We prefix these types with a capital P, such as PInteger, PMaybe a, PBool, and so forth. As previously mentioned, these form the “tags” for Plutarch Term’s, representing the type of the result of compiling and evaluating a Plutarch Script.
B.) Working with Plutarch Terms, which are values of the type Term (s :: S) (a :: PType). These are the Plutarch scripts themselves, from which we build up more complex scripts before compiling and executing them on-chain.
C.) Writing Haskell-level functions between Plutarch Terms (i.e., with types like Term s a -> Term s b). Doing so allows us to leverage Haskell’s language features and ecosystem.
D.) Efficiently Converting the functions from (C.) to Plutarch-level functions, which are of the type Term s (a :--> b). We can directly convert the functions from (C.) to Plutarch-level functions at the most naive level using plam. Additional Plutarch utilities provide for optimization opportunities.
E.) Compiling and executing the functions from (D.), targeting UPLC for on-chain usage.

As a preview, the bridge Plutarch provides between Haskell and UPLC looks something like this:

------------------------------------------------------
|                  *Haskell World*                   |
------------------------------------------------------
| Values with types like `Bool`, `Integer`, `Maybe a`|
| and regular Haskell functions like a -> b          |
------------------------------------------------------
                           ^                     |
 (functions like `plift`)--|                     |--(functions like `pconstant` or `plam`)
                           |                     |
                           |                     v         (`pcon`)
-------------------------------------------------------    |                     -------------------------------------------------------
|                  *Plutarch Term World*              | <----------------------- |                  *Plutarch Type World*              |
-------------------------------------------------------                          -------------------------------------------------------
| STLC terms; constants like `Term s PInteger` and    | -----------------------> | Types like `PInteger`, `PMaybe a`                   |
| lambdas like `Term s (PInteger :--> PBool)`         |        |                 |                                                     |
-------------------------------------------------------        (`pmatch`)        -------------------------------------------------------
                              |
                              |
                              |--(`compile`)
                              |
                              |
                              v
-------------------------------------------------------
|                    *UPLC World*                     |
-------------------------------------------------------
| Untyped lambda calculus terms. Values of type `Data`|
|                                                     |
-------------------------------------------------------

Further, you may notice two general categories of functions in Plutarch: “Haskell-level” functions between terms, and “Plutarch-level” functions as lambda terms. By convention, we will prefix the Haskell-level functions with h and the Plutarch-level lambdas with p, for example

-- This example is listed here as a preview; the unfamiliar parts will
-- be detailed below.

-- A Plutarch-level lambda term
pf :: Term s (a :--> b :--> c)
pf = undefined

-- Recovering a Haskell level function from a Plutarch level function
hf :: Term s a -> Term s b -> Term s c
hf x y = pf # x # y

Note that pf is truly just a Plutarch Term and should not be treated specially.

The remainder of this document cover the bridge between Haskell and Plutarch at a high level. It will not cover all techniques necessary to write production-ready scripts. Nor will it cover the bridge between Plutarch and UPLC beyond the minimum. Nonetheless, it should provide sufficient background to prepare the reader for further study.

Sections:-

Introduction

Overview

References

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