Do syntax with QualifiedDo.md

Do syntax with QualifiedDo and Plutarch.Monadic

There's another way of having do syntax. Though this one doesn't use a lawful monad. Instead, it uses QualifiedDo - and therefore requires GHC 9.

The Plutarch.Monadic module exports >>=, >>, and fail functions suitable to be used with QualifiedDo.

-- NOTE: REQUIRES GHC 9!
{-# LANGUAGE QualifiedDo #-}

import Plutarch.Api.Contexts
import qualified Plutarch.Monadic as P
import Plutarch.Prelude

f :: Term s (PScriptPurpose :--> PUnit)
f = plam $ \x -> P.do
  PSpending _ <- pmatch x
  ptrace "matched spending script purpose"
  pconstant ()

In essence, P.do { x; y } simply translates to x y; where x :: a -> Term s b and y :: a.

Similarly, P.do { y <- x; z } translates to x $ \case { y -> z; _ -> ptraceError <msg> }; where x :: (a -> Term s b) -> Term s b, y :: a, and z :: Term s b. Of course, if y is a fully exhaustive pattern match (e.g. singular constructor), the extra _ -> ptraceError <msg> case will not be generated at all and you'd simply get x $ \y -> z.

Finally, P.do { x } is just x.