Add Newtype exercise to calculate wattage. (purescript-contrib#273)

* Add Newtype exercise to calculate wattage.

* Change calculateWattage to use Amps and Volts and existing Watt newtype.

* Update text/chapter5.md

Co-authored-by: milesfrain <[email protected]>

* Add solution to calculateWattage exercise.

* Add Newtype exercise to calculate wattage.

* Change calculateWattage to use Amps and Volts and existing Watt newtype.

* Add solution to calculateWattage exercise.

* Update text/chapter5.md

Co-authored-by: milesfrain <[email protected]>

* Add imports to solutions to fix CI

* Directly compare wrapped values in calculateWattage test

* Change MakeWatt example to MakeCoulomb. Update exercise to ask reader
to define MakeWatt and to write calculateWattage.

* Update solutions to include Watt newtype and fix imports.

* Fix CI - must wait to import shapeBounds until solutions are reset

Other misc changes:
- No need for Columb Eq and Show
- Construct with Watt, rather than MakeWatt

Co-authored-by: milesfrain <[email protected]>
Co-authored-by: Miles Frain <[email protected]>

Author: shaunplee

exercises/chapter5/src/ChapterExamples.purs

@@ -120,10 +120,10 @@ current :: Amp
 current = calculateCurrent battery lightbulb
 -- ANCHOR_END: calculateCurrent
 
+-- ANCHOR: Coulomb
+newtype Coulomb = MakeCoulomb Number
+-- ANCHOR_END: Coulomb
+
 -- These are to enable testing. Will be explained in Ch6.
 derive newtype instance eqAmp :: Eq Amp
 derive newtype instance showAmp :: Show Amp
-
--- ANCHOR: Watt
-newtype Watt = MakeWatt Number
--- ANCHOR_END: Watt

exercises/chapter5/test/Main.purs

@@ -4,7 +4,7 @@ import Prelude hiding (gcd)
 import Test.MySolutions
 import Test.NoPeeking.Solutions  -- Note to reader: Delete this line
 
-import ChapterExamples (Amp(..), current, fromString, gcd, gcdV2, isEmpty, livesInLA, lzs, partialFunction, showPerson, showPersonV2, sortPair, takeFive, toString, unknownPerson)
+import ChapterExamples (Amp(..), current, fromString, gcd, gcdV2, isEmpty, livesInLA, lzs, partialFunction, showPerson, showPersonV2, sortPair, takeFive, toString, unknownPerson, Volt(..))
 import Data.Int (round)
 import Data.Maybe (Maybe(Just, Nothing))
 import Data.Person (Person)
@@ -109,6 +109,11 @@ Note to reader: Delete this line to expand comment block -}
           $ shapeText $ Rectangle origin 1.0 1.0
         Assert.equal Nothing
           $ shapeText $ Line origin { x: 1.0, y: 1.0 }
+    suite "Exercise Group - Newtype" do
+      test "Exercise - calculateWattage" do
+        Assert.equal 60.0
+          $ let (Watt w) = calculateWattage (Amp 0.5) (Volt 120.0)
+            in w
     suite "Exercise Group - Vector Graphics" do
       test "Exercise - area" do
         Assert.equal 50
@@ -171,4 +176,4 @@ runChapterExamples =
       Assert.equal true
         $ partialFunction true
     test "current" do
-      Assert.equal (Amp 0.003) current
+      Assert.equal (Amp 0.003) current

exercises/chapter5/test/no-peeking/Solutions.purs

@@ -2,6 +2,7 @@ module Test.NoPeeking.Solutions where
 
 import Prelude
 
+import ChapterExamples (Amp(..), Volt(..), Coulomb(..))
 import Data.Maybe (Maybe(Just, Nothing))
 import Data.Person (Person)
 import Data.Picture
@@ -101,3 +102,8 @@ Your solution should edit `shapeBounds` in `Picture.purs`.
 shapeBounds :: ShapeExt -> Bounds
 shapeBounds (Clipped pic pt w h) = intersect (bounds pic) (DataP.shapeBounds (Rectangle pt w h))
 shapeBounds (Shape shape) = DataP.shapeBounds shape
+
+newtype Watt = Watt Number
+
+calculateWattage :: Amp -> Volt -> Watt
+calculateWattage (Amp i) (Volt v) = Watt $ i * v

text/chapter5.md

@@ -449,13 +449,21 @@ This design principle is perhaps better [communicated visually](https://twitter.
 
 Note that the constructor of a newtype often has the same name as the newtype itself, but this is not a requirement. For example, unique names are also valid:
 ```haskell
-{{#include ../exercises/chapter5/src/ChapterExamples.purs:Watt}}
+{{#include ../exercises/chapter5/src/ChapterExamples.purs:Coulomb}}
 ```
 
-In this case, `Watt` is the _type constructor_ and `MakeWatt` is the _data constructor_. These constructors live in different namespaces, even when the names are identical, such as with the `Volt` example. This is true for all ADTs.
+In this case, `Coulomb` is the _type constructor_ and `MakeCoulomb` is the _data constructor_. These constructors live in different namespaces, even when the names are identical, such as with the `Volt` example. This is true for all ADTs.
 
 Another application of newtypes is to attach different _behavior_ to an existing type without changing its representation at runtime. We cover that use case in the next chapter when we discuss _type classes_.
 
+## Exercises
+
+1. (Easy) Use `newtype` to define a `Watt` unit for `Number` with `MakeWatt` as the data constructor, then define a `calculateWattage` function using this new `Watt` type and the above definitions `Amp` and `Volt`:
+```haskell
+calculateWattage :: Amp -> Volt -> Watt
+```
+A wattage in `Watt`s can be calculated as the product of a given current in `Amp`s and a given voltage in `Volt`s.
+
 ## A Library for Vector Graphics
 
 Let's use the data types we have defined above to create a simple library for using vector graphics.