Alternative to REPL for UI

[Charles-Johnson]

Instead of having a traditional read-evaluate-print-loop for interpreter UI, a possibly more conversational experience could be implemented that also would allow for asynchronous and multiple responses.

Currently the UI for the website and izia expects the user to either

• define a true expression typing let expression which either evaluates to an error message or an empty string on success
• check the normal form of expression by typing expression which evaluates to expression if irreducible or reduction is unknown or something else if reducible

This requires the user to either wait for the evaluation before entering a new command or have to insert the evaluation result in the right position in the history. This is because the evaluation result only makes sense when paired with the input.

Instead the UI could follow an alternative schema

• define a true expression just by typing expression and getting a response let expression -> result where result could be error, redundant or okay
• check the normal form of expression by typing expression ? and getting a response ((_x_ exists_such_that) expression -> _x_) -> result where result is unknown if reduction is unknown or false if irreducible or a response expression -> reduction if reduction is known.

[Charles-Johnson]

blocked by #60

[Charles-Johnson]

I think that an unknown concept will ruin everything as it's basically reinventing NULL, the billion dollar mistake.

The response to expression ? should be expression ? if the reduction is unknown

[Charles-Johnson]

This might conflict with ideas in #42 which references let in expressions and need to think how the equivalent of forget a would be handled