https://github.com/justinwoo/purescript-expect-inferred

Library for checking inferred types of values for testing
https://github.com/justinwoo/purescript-expect-inferred

purescript

Last synced: 15 days ago
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Library for checking inferred types of values for testing

• Host: GitHub
• URL: https://github.com/justinwoo/purescript-expect-inferred
• Owner: justinwoo
• License: mit
• Created: 2018-08-10T12:23:51.000Z (over 6 years ago)
• Default Branch: master
• Last Pushed: 2021-12-15T12:57:38.000Z (about 3 years ago)
• Last Synced: 2024-12-07T04:03:52.718Z (about 1 month ago)
• Topics: purescript
• Language: PureScript
• Homepage:
• Size: 5.86 KB
• Stars: 2
• Watchers: 3
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

        
# purescript-expect-inferred

[![Build Status](https://travis-ci.org/justinwoo/purescript-expect-inferred.svg?branch=master)](https://travis-ci.org/justinwoo/purescript-expect-inferred)

Library for when you want to expect that something is inferred correctly. This library is not commonly useful.

## How this works

```purs

class ExpectInferred expected actual

```

No fundeps, so both have to be determined to match anything, which means that in PureScript, you have to let-bind your intermediate actual results.

```purs

expectInferred

  :: forall expected actual

   . ExpectInferred expected actual

  => Proxy expected

  -> actual

  -> Unit

expectInferred _ _ = unit

```

## Usage

Given some class and some of its methods:

```purs

class SimpleClass a b | a -> b where

  simpleMethod :: Proxy a -> b

instance simpleInstance1 :: SimpleClass Int String where

  simpleMethod _ = "hello"

instance simpleInstance2 :: SimpleClass String Unit where

  simpleMethod _ = unit

```

We can see usage as expected:

```purs

test1 :: Unit

test1 =

  let

    expectedP = Proxy :: Proxy String

    simpleValue = simpleMethod (Proxy :: Proxy Int)

  in

    expectInferred expectedP simpleValue

```

Then, when we put the wrong thing in:

```purs

test2 :: Unit

test2 =

  let

    -- this will error correctly:

    expectedP = Proxy :: Proxy String

    -- A custom type error occurred while solving type class constraints:

    --

    --   The expected (first) and actual (second) types did not match:

    --     String

    --     Unit

    --

    -- while applying a function expectInferred

    --   of type ExpectInferred t0 t1 => Proxy t0 -> t1 -> Unit

    --   to argument expectedP

    -- while inferring the type of expectInferred expectedP

    -- in value declaration test2

    simpleValue = simpleMethod (Proxy :: Proxy String)

  in

    expectInferred expectedP simpleValue

```