type-spec: Type Level Specification by Example

[ bsd3, library, testing ] [ Propose Tags ]
Versions [RSS],,,,
Dependencies base (>=4.9 && <5), pretty (>=1.1.3 && <1.2) [details]
License BSD-3-Clause
Copyright 2016-2019 Sven Heyll
Author Sven Heyll
Maintainer sven.heyll@gmail.com
Category Testing
Home page https://github.com/sheyll/type-spec#readme
Source repo head: git clone https://github.com/sheyll/type-spec
Uploaded by SvenHeyll at 2019-01-21T18:51:07Z
Distributions LTSHaskell:, NixOS:, Stackage:
Reverse Dependencies 2 direct, 1 indirect [details]
Downloads 3834 total (13 in the last 30 days)
Rating 2.0 (votes: 1) [estimated by Bayesian average]
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Status Docs available [build log]
Last success reported on 2019-01-21 [all 1 reports]

Readme for type-spec-

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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:

     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

The output:

     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:

     There are a variety aliases for the basic combinators.
       Basic Combinators
           labels expectations using 'It'
               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 ::
        (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:

   • 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].