Type Checking

Testing for non-compilation

Libraries like shapeless which make extensive use of type-level computation and implicit resolution often need to provide guarantees that certain expressions *don't* typecheck. Testing these guarantees is supported in shapeless via the illTyped macro,

import shapeless.test.illTyped

scala> illTyped { """1+1 : Boolean""" }

scala> illTyped { """1+1 : Int""" }
<console>:19: error: Type-checking succeeded unexpectedly.
Expected some error.
illTyped { """1+1 : Int""" }

The testing library ScalaTest also has a way of checking that a snippet of code does not compile: pass it in to assertTypeError. What will happen if we combine illTyped and assertTypeError? (Hint: it's like applying a double negative.) 

import shapeless.test.illTyped

val matchedTypes = Try(assertTypeError("illTyped { \"val a: Int = 1\" }")).isSuccess
matchedTypes should be(res0)

val mismatchedTypes = Try(assertTypeError("illTyped { \"val a: String = 1\" }")).isSuccess
mismatchedTypes should be(res1)