diff --git a/checked.ts b/checked.ts
index f96e386..f2d4d31 100644
--- a/checked.ts
+++ b/checked.ts
@@ -4,6 +4,7 @@ import { Position } from "./token.ts";
 export type CheckedExpr =
     & {
         pos: Position;
+        valueType: CheckedType;
     }
     & ({ exprType: "error"; message: string } | {
         exprType: "unit";
@@ -13,7 +14,6 @@ export type CheckedExpr =
     } | {
         exprType: "int";
         value: number;
-        valueType: CheckedType;
     } | {
         exprType: "if";
         condition: CheckedExpr;
@@ -23,7 +23,6 @@ export type CheckedExpr =
         exprType: "block";
         body: CheckedExpr[];
         value: CheckedExpr | null;
-        valueType: CheckedType;
     } | {
         exprType: "call";
         subject: CheckedExpr;
diff --git a/checker.ts b/checker.ts
index 353f3d1..6bb7842 100644
--- a/checker.ts
+++ b/checker.ts
@@ -60,25 +60,117 @@ export class Checker {
     private searchTopLevelFn(statement: ParsedExpr<"fn">) {
     }
 
+    private checkStatement(expr: ParsedExpr): CheckedExpr {
+        switch (expr.exprType) {
+            case "if":
+                return this.checkIfStatement(expr);
+            case "let":
+                return this.checkLetStatement(expr);
+            case "fn":
+                return this.checkFnStatement(expr);
+            case "return":
+                return this.checkReturnStatement(expr);
+            case "loop":
+                return this.checkLoopStatement(expr);
+            case "while":
+                return this.checkWhileStatement(expr);
+            case "break":
+                return this.checkBreakStatement(expr);
+            case "continue":
+                return this.checkContinueStatement(expr);
+            default:
+                return this.checkExpr(expr);
+        }
+    }
+
+    private checkIfStatement(expr: ParsedExpr<"if">): CheckedExpr {
+        const condition = this.checkExpr(expr.condition);
+        const truthy = this.checkExpr(expr.truthy);
+        const falsy = expr.falsy ? this.checkExpr(expr.falsy) : null;
+        return {
+            pos: expr.pos,
+            exprType: "if",
+            condition,
+            truthy,
+            falsy,
+            valueType: { pos: expr.pos, typeType: "unit" },
+        };
+    }
+
+    private checkLetStatement(expr: ParsedExpr<"let">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private checkFnStatement(expr: ParsedExpr<"fn">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private checkReturnStatement(expr: ParsedExpr<"return">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private checkLoopStatement(expr: ParsedExpr<"loop">): CheckedExpr {
+        const body = this.checkExpr(expr.body);
+        return {
+            pos: expr.pos,
+            exprType: "loop",
+            body,
+            valueType: { pos: expr.pos, typeType: "unit" },
+        };
+    }
+
+    private checkWhileStatement(expr: ParsedExpr<"while">): CheckedExpr {
+        const condition = this.checkExpr(expr.condition);
+        const body = this.checkExpr(expr.body);
+        return {
+            pos: expr.pos,
+            exprType: "while",
+            condition,
+            body,
+            valueType: { pos: expr.pos, typeType: "unit" },
+        };
+    }
+
+    private checkBreakStatement(expr: ParsedExpr<"break">): CheckedExpr {
+        return { ...expr, valueType: { pos: expr.pos, typeType: "unit" } };
+    }
+
+    private checkContinueStatement(expr: ParsedExpr<"continue">): CheckedExpr {
+        return { ...expr, valueType: { pos: expr.pos, typeType: "unit" } };
+    }
+
     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" };
+                return {
+                    pos: expr.pos,
+                    exprType: "unit",
+                    valueType: { pos: expr.pos, typeType: "unit" },
+                };
             case "id":
-                throw new Error("not implemented");
+                return this.checkId(expr);
             case "int":
                 return this.checkInt(expr);
             case "if":
+                return this.checkIf(expr);
             case "block":
+                return this.checkBlock(expr);
             case "call":
+                return this.checkCall(expr);
             case "index":
+                return this.checkIndex(expr);
             case "increment":
+                return this.checkIncrement(expr as ParsedExpr<"increment">);
             case "decrement":
+                return this.checkDecrement(expr as ParsedExpr<"decrement">);
             case "unary":
+                return this.checkUnary(expr);
             case "binary":
+                return this.checkBinary(expr);
             case "assign":
+                return this.checkAssign(expr);
             default:
                 return this.errorExpr(
                     `expected expression, got '${expr.exprType}' statement`,
@@ -87,7 +179,14 @@ export class Checker {
         }
     }
 
+    private checkId(_expr: ParsedExpr<"id">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
     private checkInt(expr: ParsedExpr<"int">): CheckedExpr {
+        if (expr.value >= 1 << 15) {
+            return this.errorExpr("16-bit literal too large", expr.pos);
+        }
         return {
             pos: expr.pos,
             exprType: "int",
@@ -96,11 +195,118 @@ export class Checker {
         };
     }
 
+    private checkIf(expr: ParsedExpr<"if">): CheckedExpr {
+        const condition = this.checkExpr(expr.condition);
+        const truthy = this.checkExpr(expr.truthy);
+        if (!expr.falsy) {
+            this.errors.push({
+                pos: expr.pos,
+                message: "expected else-block in if expression",
+            });
+            return {
+                pos: expr.pos,
+                exprType: "if",
+                condition,
+                truthy,
+                falsy: null,
+                valueType: { pos: expr.pos, typeType: "unit" },
+            };
+        }
+        const falsy = this.checkExpr(expr.falsy);
+        const valueType = this.compatibleType(
+            truthy.valueType,
+            falsy.valueType,
+        );
+        return {
+            pos: expr.pos,
+            exprType: "if",
+            condition,
+            truthy,
+            falsy,
+            valueType,
+        };
+    }
+
+    private checkBlock(expr: ParsedExpr<"block">): CheckedExpr {
+        const body = expr.body.map((statement) =>
+            this.checkStatement(statement)
+        );
+        if (expr.value) {
+            const value = this.checkExpr(expr.value);
+            return {
+                pos: expr.pos,
+                exprType: "block",
+                body,
+                value,
+                valueType: value.valueType,
+            };
+        } else {
+            return {
+                pos: expr.pos,
+                exprType: "block",
+                body,
+                value: null,
+                valueType: { pos: expr.pos, typeType: "unit" },
+            };
+        }
+    }
+
+    private checkCall(expr: ParsedExpr<"call">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private checkIndex(expr: ParsedExpr<"index">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private checkIncrement(expr: ParsedExpr<"increment">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private checkDecrement(expr: ParsedExpr<"decrement">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private checkUnary(expr: ParsedExpr<"unary">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private checkBinary(expr: ParsedExpr<"binary">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private checkAssign(expr: ParsedExpr<"assign">): CheckedExpr {
+        throw new Error("not implemented");
+    }
+
+    private compatibleType(
+        a: CheckedType,
+        b: CheckedType,
+        reversed = false,
+    ): CheckedType {
+        if (a.typeType === "error") {
+            return a;
+        } else if (b.typeType === "error") {
+            return b;
+        } else if (a.typeType === "unit" && b.typeType === "unit") {
+            return a;
+        } else if (a.typeType === "u16" && b.typeType === "u16") {
+            return a;
+        } else if (a.typeType === "u16" && b.typeType === "i16") {
+            return a;
+        } else if (!reversed) {
+            return this.compatibleType(b, a, true);
+        } else {
+            throw new Error("not implemented");
+        }
+    }
+
     private errorExpr(message: string, pos: Position): CheckedExpr {
         this.errors.push({ message, pos });
         return {
             pos,
             exprType: "error",
+            valueType: { pos, typeType: "unit" },
             message,
         };
     }