add 'makeEffect' template stub

agora/Agora/Effect.hs

@@ -5,4 +5,33 @@ Description: Helpers for constructing effects
 
 Helpers for constructing effects.
 -}
-module Agora.Effect () where
+module Agora.Effect (makeEffect) where
+
+import Plutarch.Api.V1 (PScriptPurpose (PSpending), PTxInfo, PTxOutRef, PValidator)
+import Plutarch.Internal (punsafeCoerce)
+import Plutarch.Monadic qualified as P
+
+--------------------------------------------------------------------------------
+
+-- | Helper 'template' for creating effect validator.
+makeEffect ::
+  forall (datum :: PType) (s :: S).
+  PIsData datum =>
+  (Term s datum -> Term s PTxOutRef -> Term s PTxInfo -> Term s POpaque) ->
+  Term s PValidator
+makeEffect f =
+  plam $ \datum _redeemer ctx' -> P.do
+    ctx <- pletFields @'["txInfo", "purpose"] ctx'
+    txInfo' <- plet ctx.txInfo
+
+    let datum' :: Term _ datum
+        datum' = pfromData $ punsafeCoerce datum
+
+    PSpending txOutRef <- pmatch $ pfromData ctx.purpose
+    txOutRef' <- plet (pfield @"_0" # txOutRef)
+
+    -- TODO: Here, check that a *single* GAT is burned.
+
+    f datum' txOutRef' txInfo'
+
+--------------------------------------------------------------------------------