Explicit local bindings for let/where

src/Check/Core.hs

@@ -165,6 +165,9 @@ definitionUnsafeBaseReason localNames allowedExternalFacts bound ast = case ast
   TStem _ -> Just "uses raw t directly"
   TFork _ _ -> Just "uses raw t directly"
   SLambda args body -> definitionUnsafeBaseReason localNames allowedExternalFacts (foldr Set.insert bound args) body
+  SLet name val body ->
+    definitionUnsafeBaseReason localNames allowedExternalFacts bound val
+      <|> definitionUnsafeBaseReason localNames allowedExternalFacts (Set.insert name bound) body
   SEmpty -> Nothing
   SImport _ _ -> Nothing
 
@@ -187,6 +190,7 @@ astFreeRefs candidates ast = case ast of
   TStem inner -> astFreeRefs candidates inner
   TFork left right -> astFreeRefs candidates left ++ astFreeRefs candidates right
   SLambda args body -> astFreeRefs (foldr Set.delete candidates args) body
+  SLet name val body -> astFreeRefs candidates val ++ astFreeRefs (Set.delete name candidates) body
   SEmpty -> []
   SImport _ _ -> []
 
@@ -584,6 +588,14 @@ lowerExprKnownAgainst expr expected = case (expr, viewExprAsType expected) of
   (SApp (SApp (SVar "err" _) value) rest, Just (VTResult errView _)) ->
     lowerUnshadowedConstructor "err" expr expected $
       lowerResultConstructor expected (viewTypeToExpr errView) value rest
+  (SLet name value body, _) -> do
+    (valueSym, valueNodes, _) <- lowerExprKnown value
+    recordDebugName valueSym name
+    bodyResult <- withLocalAlias name valueSym (lowerExprKnownAgainst body expected)
+    let (bodySym, bodyNodes, bodyKnown) = bodyResult
+    pure (bodySym, valueNodes ++ bodyNodes, bodyKnown)
+  -- Hand-written immediately-applied lambda (not compiler output; let/where
+  -- now emit SLet). Kept for source that relies on alias semantics.
   (SApp (SLambda [name] body) value, _) -> do
     (valueSym, valueNodes, _) <- lowerExprKnown value
     bodyResult <- withLocalAlias name valueSym (lowerExprKnownAgainst body expected)
@@ -658,6 +670,14 @@ lowerExprKnown TLeaf = do
 lowerExprKnown (SList items) = do
   (sym, nodes, view, _) <- lowerListLiteral items
   pure (sym, nodes, Just view)
+lowerExprKnown (SLet name value body) = do
+  (valueSym, valueNodes, _) <- lowerExprKnown value
+  recordDebugName valueSym name
+  bodyResult <- withLocalAlias name valueSym (lowerExprKnown body)
+  let (bodySym, bodyNodes, bodyKnown) = bodyResult
+  pure (bodySym, valueNodes ++ bodyNodes, bodyKnown)
+-- Hand-written immediately-applied lambda (not compiler output; let/where
+-- now emit SLet). Kept for source that relies on alias semantics.
 lowerExprKnown (SApp (SLambda [name] body) value) = do
   (valueSym, valueNodes, known) <- lowerExprKnown value
   bodyResult <- withLocalAlias name valueSym (lowerExprKnown body)