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Monkey lang

An interpreter written in Go, based on the https://interpreterbook.com.

Running the REPL locally

You just need to have go installed. Then run:

git clone https://github.com/juandspy/monkey-lang.git
go run main.go

and start writing Monkey lang code. The results of the statements will be printed in the stdout. For example:

>> 3 * 7
21
>> let x = 3 * 7
>> x
21

Note that some statements like variable bindings don't print anything in the stdout.

Language specs

Types

There are 5 types supported:

  • Booleans: true or false
  • Integers: 1, -1, 12345...
  • Strings: "Hello World"
  • Arrays: [1, 2, 3]. You can access a given position of an array by using indexes: [1, 2, 3][1] or myArray[1].
  • Hashes: {"a": 1, 5: "test", true: "bool"}

Operators

Prefix expressions

  • Bang (!): it takes any input and returns the opposite. For example !true = false and !5 = false, as 5 acts as "truthy". However, !!5 would be true as it's the same as !false.
  • Minus (-): changes the sign of an integer e.g. -5.

Infix expressions

  • Arithmetic expressions: (1 + 2) * 3 / 4
  • Comparisons: 3 != 2
  • Conditionals: if (3 == 3) {"equals"} else {"not equals"}

Variables

You can define a variable by using let statements, e.g. let x = 3. You can also bind expressions: let x = 3 * 7.

Functions

You can bind functions to variables using the let statement:

let sum = fn(x, y) {return x + y}; sum(1, 2)

You can also build recursive functions:

>> let fib = fn(n) { if (n < 2) { return n; } else {return fib(n-1) + fib(n-2); } }
>> fib(20)
6765

Builtin functions

There is a set of builtin functions available which are defined in builtins.go:

  • len: returns the length of a string or array.
  • first: returns the first element of an array.
  • last: returns the last element of an array.
  • rest: returns all the elements except the first one.
  • push: appends an item to an array.
  • puts: output to stdout.