Created
July 15, 2022 12:57
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Representing money in PureScript using phantom types and symbols
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| newtype Money currency = Money Number -- Constructor is not exported | |
| sumMoney :: forall currency. Money currency -> Money currency -> Money currency | |
| sumMoney (Money a) (Money b) = Money (a `add` b) | |
| infixl 6 sumMoney as + | |
| multiplyMoney :: forall currency. Number -> Money currency -> Money currency | |
| multiplyMoney multiplier (Money a) = Money (multiplier `mul` a) | |
| infixl 7 multiplyMoney as * | |
| eur :: Number -> Money "EUR" | |
| eur = Money | |
| usd :: Number -> Money "USD" | |
| usd = Money | |
| ten = eur 7.0 + 2.0 * eur 3.0 | |
| -- illegal = eur 1.0 + usd 1.0 -- Does not type-check: Could not match type "USD" with type "EUR" | |
| -- | A somewhat dangerous helper constructor function. Optional so can (should?) be left out, but interesting because | |
| -- | it avoids extending the module with new smart constructors like `eur` and `usd` for additional currencies. | |
| money :: forall (currency :: Symbol). Number -> Money currency | |
| money = Money | |
| -- Example usage of `money`, without a dedicated yen smart constructor: | |
| yen1000 = money 1000.0 :: Money "YEN" -- Force instantiation of the currency type variable using a type annotation |
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