feat: a basic visual programming linter

src/Data/Dataflow/Expression.purs

@@ -19,9 +19,11 @@ module Lunarbox.Data.Dataflow.Expression
   , wrappers
   , everywhereOnExpressionM
   , everywhereOnExpression
+  , foldExpression
   ) where
 
 import Prelude
+import Control.Monad.Writer (execWriter, tell)
 import Data.Identity (Identity(..))
 import Data.List (List(..), foldr, (:))
 import Data.Map as Map
@@ -89,6 +91,10 @@ everywhereOnExpressionM f = go'
 everywhereOnExpression :: forall l. (Expression l -> Expression l) -> Expression l -> Expression l
 everywhereOnExpression mapper = unwrap <<< everywhereOnExpressionM (Identity <<< mapper)
 
+-- | Accumulate a value running trough all the layers of an expression
+foldExpression :: forall l m. Monoid m => (Expression l -> m) -> Expression l -> m
+foldExpression f = execWriter <<< everywhereOnExpressionM (\expr -> tell (f expr) $> expr)
+
 -- Takes a list of argument names and a body and creates the body of a function
 functionDeclaration :: forall l. l -> Expression l -> List VarName -> Expression l
 functionDeclaration = foldr <<< Lambda

src/Data/Dataflow/Expression/Lint.purs

@@ -0,0 +1,28 @@
+module Lunarbox.Data.Dataflow.Expression.Lint where
+
+import Prelude
+import Data.Array as Array
+import Lunarbox.Data.Dataflow.Expression (Expression(..), VarName, foldExpression)
+
+data LintError l
+  = UnusedDeclaration VarName l
+
+-- | Collect linting errors inside an expression
+lint :: forall l. Expression l -> Array (LintError l)
+lint = foldExpression go
+  where
+  go (Let location name _ body)
+    | Array.null (references name body) =
+      [ UnusedDeclaration name location
+      ]
+
+  go a = []
+
+-- | Get all the places a variable is referenced in
+references :: forall l. VarName -> Expression l -> Array l
+references target = foldExpression go
+  where
+  go (Variable location name)
+    | target == name = [ location ]
+
+  go _ = []