builtin: Add rand and strings functions

[juliaogris]

Add random number generation functions:

• rand n returns an integer in the half open interval [0,n)
• rand1 returns an float64 in the half open interval [0,1)

Add string functions upper, lower, index, trim,
startswith, endswith, replace.

This functions are mapped to Go's math/rand package. It seems that
tinygo still requires seeding (tested).

Add a demos for clarity.

Minor bug fix for non 2xx source response: don't load random HTLM,
write error to console instead.

https://evy-lang--82-r0m72zv4.web.app/#source=samples/rand.evy&show=.read
https://evy-lang--82-r0m72zv4.web.app/#strings

[github-actions]

firebase-deployment: https://evy-lang--82-r0m72zv4.web.app (5348d79)

[juliaogris]

From Mali: hey mum, the sample code would be better like this:

while true
  print "╔════════════════════════╗"
  print "║ Guess my number (1-10) ║"
  print "╚════════════════════════╝"

  n := 1 + (rand 10)
  guess := readn

  while guess != n
    if guess > 10
      print guess "Guess higher (1-10)."
    else if guess < 1
      print guess " Guess lower (1-10)."
    else if guess < n
       print guess "Guess higher."
    else 
       print  guess "Guess lower."
    end
    guess = readn
  end
  print guess "You got it 🥳."
  print
end

// readn reads a number. 
// If you enter a string that is not number
func readn:num
  while true
    str := read
    n := str2num str
    if !err
      return n
    end
    print "💣 '"+str+"'is not a number. Try again."
  end
  return -1
end

[camh-]

🦚 LGTM

I noticed when I tried the guessing game that the input line was not focused, so I typed a number (nothing happened) and I pressed enter, and that stopped the program because the Stop button was still focused. Perhaps calling read should focus on the input?

Also, I guessed the right number 3 times, so I think I deserve a cookie.

[camh-]

should this be "Guess lower" and the next "Guess higher"?

[juliaogris]

🍪
thanks for the review!

[juliaogris]

I fixed this up.
Your comment re input focus is a really good one. also fixed.