| add :: Money -> Money -> Maybe Money | |
| add m1 m2 = | |
| if currency m1 != currency m2 -- If the two currencies are different | |
| then Nothing -- * Return an empty result | |
| else Just $ Money { -- * Else return a new Money instance | |
| amount = amount m1 + amount m2, -- which is the sum of the two inputs | |
| currency = currency m1 } |
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