Home > The Interpreter > The Interpreter, Chapter 4

The Interpreter, Chapter 4

The continuing story of one man’s quest to write an interpreter.

Most of the basic data types — numbers, strings, arrays, lists, hashes — were supported. Next, the interpreter needed control structures. But before those could be added, they needed some sort of support. So Jackson created blocks.

Blocks began as chunks of code in between curly brackets. They were pretty simple. Getting them to work was a bit tricky, but nothing too difficult. Even before control structures were implemented, blocks still proved useful. They could be packaged up, and the same code could be called multiple times.

Jackson used this to implement if statements and while loops. To keep control structures as pure as possible, he treated if statements and while loops as functions. While loops became functions that took two blocks, one for the condition and one for the body. If statements became functions that took either two or three blocks, depending on whether there was an “else” block. After blocks were implemented, if statements and while loops were easy.

Now that blocks were possible, they could be used not just for control structures but also could be passed as arguments into functions. The first such function was the times() function, which took two arguments: a number and a block. It would execute the block a number of times equal to the given number, and it looked like 10 times { do stuff }.

For blocks to really achieve their full potential, they needed to be able to take variables. Implementing this required making the internal data structure a good deal more complicated. But when it was finished, the interpreter was all the more powerful.

At last, Jackson’s programming language was Turing complete. He successfully implemented a program to calculate every prime number from 1 to 100.

Chapter 5

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Categories: The Interpreter
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  1. July 7, 2010 at 4:01 am

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