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Type System

Perry erases types at compile time, similar to how tsc removes type annotations when emitting JavaScript. However, Perry also performs type inference to generate efficient native code.

Type Inference

Perry infers types from expressions without requiring annotations:

{{#include ../../examples/language/type_system.ts:inference-basics}}

Inference works through:

  • Literal values: 5 → number, "hi" → string
  • Binary operations: a + b where both are numbers → number
  • Variable propagation: if x is number, then let y = x is number
  • Method returns: "hello".trim() → string, [1,2].length → number
  • Function returns: user-defined function return types are propagated to callers
{{#include ../../examples/language/type_system.ts:inference-function}}

Type Annotations

Standard TypeScript annotations work:

{{#include ../../examples/language/type_system.ts:annotations}}

Utility Types

Common TypeScript utility types are erased at compile time (they don't affect code generation):

{{#include ../../examples/language/type_system.ts:utility-types}}

These are all recognized and erased — they won't cause compilation errors.

Generics

Generic type parameters are erased:

{{#include ../../examples/language/type_system.ts:generics}}

At runtime, all values are NaN-boxed — the generic parameter doesn't affect code generation.

Type Checking with --type-check

For stricter type checking, Perry can integrate with Microsoft's TypeScript checker:

perry file.ts --type-check

This resolves cross-file types, interfaces, and generics via an IPC protocol. It falls back gracefully if the type checker is not installed.

Without --type-check, Perry relies on its own inference engine, which handles common patterns but doesn't perform full TypeScript type checking.

Union and Intersection Types

Union types are recognized syntactically but don't affect code generation:

{{#include ../../examples/language/type_system.ts:union-narrowing}}

Use typeof checks for runtime type narrowing.

Type Guards

{{#include ../../examples/language/type_system.ts:type-guards}}

The value is string annotation is erased, but the typeof check works at runtime.

Next Steps