General feedback

Review the tutorial and interpreter entry points first, then separate the notes into concrete issues such as wording, navigation, display, persistence, and input behavior. Select one narrowly scoped item, define its expected result from the feedback, and verify that result in the Lambda Explorer experience.

I met you at the JS meetup last week and you showed me Lambda Explorer.

I'm surprised it got no attention on Hacker News when you posted it 4 years ago. Maybe it sucked back then. This sounds exactly like something they would like and it's better than every other lambda calculus interpreter/tutorial I found in a google search.

I finished it up to the 3 challenge problems (still working on them), here are my notes:

• Problem 3 - comprimise -> compromise
• Dark mode?
• Back/forward arrows too small, hard to find
• The "leftmost outermost" beta-reduction description is confusing as hell and got me tripped up trying to understand the Y-combinator reduction, in particular why (λg.(λx.g(xx))a reduces to λg.g(aa) and not λx.a(xx). A stackoverflow comment that helped me a lot with it was https://stackoverflow.com/a/34305160 because it showed the application order in boldface within each step.
• Problem 25 - I would prefer that the representations of 1 through 4 were hidden. 0 probably should be described in the right half as being identical to FALSE. I covered the right half of the screen for this one.
• Possible to use _ as an alias for "the last thing the interpreter returned" like in python?
• Refreshing saves tutorial progress but not variable declarations

Extremely nitpicky things that you don't have to fix at all

• Shorten FALSE -> F and TRUE -> T, somewhat easier to read and use. I used this shorthand when working with them on scratch paper.
• I was expecting the interpreter to tell me that λa.(a FALSE) TRUE was the Church NOT like it does for Numerals and Booleans. But then again, I guess it's consistent in that it only does it for things we usually think of as primitives.
• λε₁.ε₁ errors out. I know we're only supposed to use a-z and a number, but it is something that the interpreter outputs, so people will probably try to copy paste it as an input while playing around with it.

Again, these are all nitpicks and you did a great job! I don't think I would have bothered learning lambda calculus if the interpreter were not this fun to play with.