Simply Typed Lambda Calculus
The language we'll be using is a small extension of the simply typed lambda calculus, with an ML-like syntax. It's a very simple language so that the amount of syntax you need to know is small - here's an overview of what you can do:

The two primitive types are Int:
(13 - 2) * 4 - 6 % 3
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and Bool:
false or (true and false)
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You can set local variables using a let statement:
let x = 1 in x + 1
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Remember that the variable you define is only in scope for whatever code you write after in! If you want to define multiple variables, you can use multiple let statements:
let x = 3 in let y = 5 in x - y
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You can define a function using the fun keyword:
fun (x: Bool) -> if x then 1 else 0
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Note that each argument must be wrapped in parentheses and be annotated with a type. You can add multiple arguments to a function, which will automatically be curried:
let add = fun (x: Int) (y: Int) -> x + y in
let addone = add 1 in
addone 20
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To conditionally evaluate some code, you can use an if then/else expression
if true then 1 else 0
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Finally, there is also a built in record data structure, which maps keys to values:
let info = {age=30, is_tall=false} in info.age
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That's all there is to it! Feel free to play around in the REPL on the right, or try and write some more complicated programs in the text editor. Once you feel comfortable with the syntax, pick a type feature from the dropdown to start learning about types!
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