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murdho's solution

to Space Age in the Haskell Track

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Instructions
Test suite
Solution

Given an age in seconds, calculate how old someone would be on:

  • Earth: orbital period 365.25 Earth days, or 31557600 seconds
  • Mercury: orbital period 0.2408467 Earth years
  • Venus: orbital period 0.61519726 Earth years
  • Mars: orbital period 1.8808158 Earth years
  • Jupiter: orbital period 11.862615 Earth years
  • Saturn: orbital period 29.447498 Earth years
  • Uranus: orbital period 84.016846 Earth years
  • Neptune: orbital period 164.79132 Earth years

So if you were told someone were 1,000,000,000 seconds old, you should be able to say that they're 31.69 Earth-years old.

If you're wondering why Pluto didn't make the cut, go watch this youtube video.

Hints

In this exercise, we provided the definition of the algebric data type named Planet. You need to implement the ageOn function, that calculates how many years old someone would be on a Planet, given an age in seconds.

Your can use the provided signature if you are unsure about the types, but don't let it restrict your creativity:

ageOn :: Planet -> Float -> Float

Getting Started

For installation and learning resources, refer to the exercism help page.

Running the tests

To run the test suite, execute the following command:

stack test

If you get an error message like this...

No .cabal file found in directory

You are probably running an old stack version and need to upgrade it.

Otherwise, if you get an error message like this...

No compiler found, expected minor version match with...
Try running "stack setup" to install the correct GHC...

Just do as it says and it will download and install the correct compiler version:

stack setup

Running GHCi

If you want to play with your solution in GHCi, just run the command:

stack ghci

Feedback, Issues, Pull Requests

The exercism/haskell repository on GitHub is the home for all of the Haskell exercises.

If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue. We'll do our best to help you!

Source

Partially inspired by Chapter 1 in Chris Pine's online Learn to Program tutorial. http://pine.fm/LearnToProgram/?Chapter=01

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

Tests.hs

{-# OPTIONS_GHC -fno-warn-type-defaults #-}
{-# LANGUAGE RecordWildCards #-}

import Data.Foldable     (for_)
import Data.Function     (on)
import Test.Hspec        (Spec, describe, it, shouldBe)
import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)

import SpaceAge (Planet(..), ageOn)

main :: IO ()
main = hspecWith defaultConfig {configFastFail = True} specs

specs :: Spec
specs = describe "ageOn" $ for_ cases test
  where
    -- Here we used `fromIntegral`, `fromRational` and `toRational` to
    -- generalize the test suite, allowing any function that takes a
    -- `Planet` and a number, returning an instance of `Real`.
    test Case{..} = it description $ expression `shouldBeAround` expected
      where
        expression = fromRational
                   . toRational
                   . ageOn planet
                   . fromIntegral
                   $ seconds
        shouldBeAround = shouldBe `on` roundTo 2
        roundTo n = (/ 10 ^ n) . fromIntegral . round . (* 10 ^ n)

data Case = Case { description :: String
                 , planet      :: Planet
                 , seconds     :: Integer
                 , expected    :: Double
                 }

cases :: [Case]
cases = [ Case { description = "Earth"
               , planet      = Earth
               , seconds     = 1000000000
               , expected    = 31.69
               }
        , Case { description = "Mercury"
               , planet      = Mercury
               , seconds     = 2134835688
               , expected    = 280.88
               }
        , Case { description = "Venus"
               , planet      = Venus
               , seconds     = 189839836
               , expected    = 9.78
               }
        , Case { description = "Mars"
               , planet      = Mars
               , seconds     = 2329871239
               , expected    = 39.25
               }
        , Case { description = "Jupiter"
               , planet      = Jupiter
               , seconds     = 901876382
               , expected    = 2.41
               }
        , Case { description = "Saturn"
               , planet      = Saturn
               , seconds     = 3000000000
               , expected    = 3.23
               }
        , Case { description = "Uranus"
               , planet      = Uranus
               , seconds     = 3210123456
               , expected    = 1.21
               }
        , Case { description = "Neptune"
               , planet      = Neptune
               , seconds     = 8210123456
               , expected    = 1.58
               }
        ]
module SpaceAge (Planet(..), ageOn) where

data Planet = Mercury
            | Venus
            | Earth
            | Mars
            | Jupiter
            | Saturn
            | Uranus
            | Neptune

ageOn :: Fractional a => Planet -> a -> a
ageOn planet seconds = seconds / (earthYearInSeconds * orbitalPeriodInEarthYears planet)

earthYearInSeconds :: Fractional a => a
earthYearInSeconds = 31557600

orbitalPeriodInEarthYears :: Fractional a => Planet -> a
orbitalPeriodInEarthYears Mercury = 0.2408467
orbitalPeriodInEarthYears Venus   = 0.61519726
orbitalPeriodInEarthYears Earth   = 1
orbitalPeriodInEarthYears Mars    = 1.8808158
orbitalPeriodInEarthYears Jupiter = 11.862615
orbitalPeriodInEarthYears Saturn  = 29.447498
orbitalPeriodInEarthYears Uranus  = 84.016846
orbitalPeriodInEarthYears Neptune = 164.79132

What can you learn from this solution?

A huge amount can be learnt from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that I could read more about to develop my understanding?