I'm making an engineering language where just about everything is an expression. Lately the most interesting thing to me is the juxtapose operator, i.e. if you stick two expressions next to each other without whitespace, they are considered juxtaposed. Initially juxtapose was just going to be for math/multiplication, but I've also decided to make function calling handled via juxtapose as well (since it lets me get rid of several types of syntax and replace them with pure expression handling)

Some interesting examples:

• since the quotes delimit the string, you don't need the parenthesis

  printl'Hello, World!'
  

• though sometimes you need to disambiguate with parenthesis

  s = "Hello, World!"
  printl(s)
  

• technically you can wrap either operand, so long as they touch

  (printl)s  // though this is bad style for function calls
  

• this has a neat consequence that string prefixes are just functions, and work pretty seamlessly

  mypath = p"this/is/some/path/object"
  myregex = re"[^i*&2@]"
  myphonetics = ipa"ɛt vɔkavit dɛus aɾidam tɛɾam kɔngɾɛgatsiɔnɛskwɛ"
  

  p, re, and ipa are all just ordinary functions

• some basic math examples

  x = 3
  y = 2x
  z = (2+3y)(x*2)
  

• complex numbers/quaternions are pretty seamless

  1 + 2i
  1 + 2i + 3j + 4k
  

• also physical units will be first class citizens, and fit in pretty nicely with juxtapose

  15kg
  7(kg) * 10(m/s/s)
  25(N/m^2) + 15(Pa)
  1500(W) / 10(A)
  5(A) * 2(Ω)
  8(m*s^-1) / 2(s)
  40(N*m) * 10(rad)
  1000(m^3) * 2(kg/m^3)
  

• Where it gets really wacky/hard to parse is something like this

  sin(x)^2 + cos(x)^2    // => (sin(x))^2 + (cos(x))^2
  

  depending on the types of sin and cos different things can happen. By default sin/cos are functions, so the function call happens first, but if the user redefined them or constructed an identically formatted expression where they are numeric, then the exponent should happen first

  s = 10
  c = 20
  s(x)^2 + c(x)^2    // => s(x^2) + c(x^2)
  

  but that's all just a problem for the compiler

Been working on and off on an interpreter/compiler written in python. Pretty slow going though.