diff --git a/checked.ts b/checked.ts
index 6904c7c..7d997d1 100644
--- a/checked.ts
+++ b/checked.ts
@@ -13,6 +13,7 @@ export type CheckedExpr =
     } | {
         exprType: "int";
         value: number;
+        valueType: CheckedType;
     } | {
         exprType: "if";
         condition: CheckedExpr;
@@ -88,6 +89,8 @@ export type CheckedType =
         typeType: "unit";
     } | {
         typeType: "u16";
+    } | {
+        typeType: "i16";
     } | {
         typeType: "pointer";
         subject: CheckedType;
diff --git a/checker.ts b/checker.ts
index 8c1c402..006e755 100644
--- a/checker.ts
+++ b/checker.ts
@@ -1,6 +1,6 @@
 import { CheckedExpr, CheckedType } from "./checked.ts";
 import { ParsedExpr } from "./parsed.ts";
-import { CompileError } from "./token.ts";
+import { CompileError, Position } from "./token.ts";
 
 export type SymbolValue = {
     id: number;
@@ -51,5 +51,39 @@ export class Checker {
 
     private searchTopLevelFn(statement: ParsedExpr & { exprType: "fn" }) {
     }
-}
 
+    private checkExpr(expr: ParsedExpr): CheckedExpr {
+        switch (expr.exprType) {
+            case "error":
+                return this.errorExpr(expr.message, expr.pos);
+            case "unit":
+                return { pos: expr.pos, exprType: "unit" };
+            case "id":
+                throw new Error("not implemented");
+            case "int":
+            case "if":
+            case "block":
+            case "call":
+            case "index":
+            case "increment":
+            case "decrement":
+            case "unary":
+            case "binary":
+            case "assign":
+            default:
+                return this.errorExpr(
+                    `expected expression, got '${expr.exprType}' statement`,
+                    expr.pos,
+                );
+        }
+    }
+
+    private errorExpr(message: string, pos: Position): CheckedExpr {
+        this.errors.push({ message, pos });
+        return {
+            pos,
+            exprType: "error",
+            message,
+        };
+    }
+}