Type Level Specification by Example
|LTS Haskell 20.15:||0.4.0.0|
|Stackage Nightly 2023-03-22:||0.4.0.0|
|Latest on Hackage:||0.4.0.0|
Maintained by email@example.com
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Module documentation for 0.4.0.0
A tiny EDSL to write type-level-unit tests.
A small example:
import Test.TypeSpec main :: IO () main = print spec0 spec :: Expect "Expect something..." (Int `Isn't` Bool) spec = Valid
This will output:
Valid: Expect something... (✓ Different)
Using the operators from TypeSpecCrazy:
specCrazy :: "Higher kinded assertions" ########################### "ShouldBe accepts types of kind * -> *" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ShouldBe Maybe Maybe -* ShouldBe   -* ShouldBe (->) (->) specCrazy = Valid main = print specCrazy
Valid: Higher kinded assertions ShouldBe accepts types of kind * -> * (✓ Equal) (✓ Equal) (✓ Equal)
If you like Lisp, this might be for you:
type ALot = 1000 specAliases :: (Explain "There are a variety aliases for the basic combinators." (Context "Basic Combinators" (Describe "Context" (It "labels expectations using 'It'" (Describe "Describe" (It's "an alias for It, just like They" (It's "time for the first assertion" (1000 `Is` ALot)))))))) specAliases = Valid main = print specAliases
This will output:
Valid: There are a variety aliases for the basic combinators. Basic Combinators Context labels expectations using 'It' Describe an alias for It, just like They time for the first assertion (✓ Equal)
The key feature is that the compiler checks the assertions and expectations made in a ‘TypeSpec’ and right away rejects invalid types.
When compiling this example:
specFailing :: TypeSpec (It "counts the number of elements in a tuple" (Count ((),(),()) `ShouldBe` 4)) specFailing = Valid type family Count a :: Nat where Count (a,b) = 2 Count (a,b,c) = 3 Count (a,b,c,d) = 4
The compiler complains:
error: • counts the number of elements in a tuple Expected type: 3 Actual type: 4 • In the expression: Valid In an equation for ‘specFailing’: specFailing = Valid
After all, with
TypeError GHC is quite a test-runner.
If you accept to defer type checking and have invalid specs checked during test execution, use (should-not-typecheck)[https://github.com/CRogers/should-not-typecheck].