Haskell implementation of a simple Scheme compiler, based on [this tutorial] (https://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours).

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Follow-on Projects

I've finished the tutorial and it works! Yay, I built a language! :)

... What else can I do?

Comments

• Scheme comments start with ; and go to the end of the line
• Effectively they are treated the same as a space (delimiter between expressions)

To do:

• Modify the spaces parser to become spacesOrComments
• Deal with spaces at start of input, which cause exceptions in the interpreter
  • Parse optional spacesOrComments at start of input (in REPL and loaded files)
  • For the REPL, use Maybe to deal with completely blank lines
    • This means I can hit Enter a few times without getting error messages back!
    • REPL only. When loading a file, you kinda want an error if it's empty.

History

• In the REPL, I'm finding it annoying not to be able to hit the up arrow! How to implement this?
• Need to read a character at a time rather than a line at a time.
  • Should it accept up arrow at start of line only, or any time?
• Store each entered expression in a history list
  • Add to the list each time an expression is evaluated
• Up and down arrows let you navigate the history list
  • How to detect these keys?
• Each time a character is entered, add it to the current input string
• If character is '\n' then add it to the history list and evaluate it
• How to store history list?
  • Arg to a recursive function?
  • Mutable variable?
• Found a package to do all of this! Haskeline is used in Elm REPL.

To do:

• Install Haskeline
• Set up basic version from docs example & make sure tests still pass
• Handle Ctrl-C like Elm REPL. If text has been entered, just start a new line. If no text, quit.

Tail Call Optimisation

• Interesting. TCO kind of fascinates me for some reason.
• Detect tail calls
• Optimise tail calls
  • Can't easily do tail recursion for the eval function, because its recursion needs to follow the structure of the Scheme code.
  • But can make apply tail recursive and use it as a trampoline
  • 3x reduction in memory usage but still eventually get stack overflow
• Completely eliminate stack overflow for recursive functions

Macro expansion

• Should I try to do this?
• Never used them, could be a good way to learn it.
• Could be a bit deep and confusing