Prefer matching on pmatch result immediately.md

imports

module Plutarch.Docs.PreferMatchingOnResult (this, this') where 
import Plutarch.Prelude
import Plutarch.Api.V1 (PScriptPurpose (PSpending, PMinting, PRewarding, PCertifying))

Prefer pattern matching on the result of pmatch immediately

You should always try and pattern match on the result of pmatch immediately. This is because the semantics of pmatch will make anything you write before the pattern match be inlined for every single branch:

this :: Term s (PScriptPurpose :--> PInteger)
this = plam $ \x -> pmatch x $ \l ->
  plet (1 + 2) $ \i -> case l of
    PMinting _ -> i + 3
    PSpending _ -> i + 4
    PRewarding _ -> i + 5
    PCertifying _ -> i + 6

Notice how the above code plets a computation before matching on l, the pmatch result. This will make the plet $ 1 + 2 $ \i -> i + <something> be inlined in every branch of your pattern match! That is, not only will it compute the 1 + 2 every time, it will also plet it, which introduced an extra lambda, only to immediately apply the lambda!

You should always match on the result immediately, whenever possible:

this' :: Term s (PScriptPurpose :--> PInteger)
this' = plam $ \x -> plet  (1 + 2) $ \i ->
  pmatch x $ \case
    PMinting _ -> i + 3
    PSpending _ -> i + 4
    PRewarding _ -> i + 5
    PCertifying _ -> i + 6

This applies much the same with do syntax (whether with TermCont or with QualifiedDo). Try to use inline partial pattern matching (e.g PMinting _ <- pmatch x), or pattern match on the very next line (e.g l <- pmatch x; case l of ...).