Building a Calculator
Now that you have some experience working with sum types and implementing inductively defined data structures, let’s put that knowledge into action and build a more sophisticated application. In this section, you’ll build a small program to parse and evaluate arithmetic expressions in prefix notation. Before you start building the program, let’s look at some examples of the application running:
| | λ run "+ 3 5" |
| | "The answer is: 8" |
| | λ run "/ 16 4" |
| | "The answer is: 4" |
| | λ run "* 2 / 16 4" |
| | "The answer is: 8" |
| | λ run "- 10 + 1 * 2 / 8 4" |
| | "The answer is: 5" |
As you can see in the examples, you’ll need to support four different operations: addition, subtraction, multiplication, and division. Whenever you ...
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