6. Various flow control

If-expression

Squiggle has keywords if and else, but they're actually like JavaScript's ternary operator (p ? x : y), with the exception that Squiggle throws an error if the value being checked is not a boolean.

Note that the else clause is not optional. It must be present in all if-expressions.

console.log(
  if true then
    3
  else
    4
  end
)

This will log 3 to the console.

If you have more than two cases, you can use elseif:

def numberToEnglish(n) do
  if n == 1 then "one"
  elseif n == 2 then "two"
  elseif n == 3 then "three"
  elseif n == 4 then "four"
  else "some other number"
  end
end

console.log(numberToEnglish(2))

This would be better written with a match-expression, but more on that in a later chapter.

Try-expression

The keyword try is used to catch exceptions. Unlike JavaScript, Squiggle's try produces values that can be used, rather than merely executing statements.

It is used before an expression, like try f() or try foo.bar(). The result of a try expression is a length 2 array that looks like either ["ok", value] or ["fail", error]. You can manually inspect these values using brackets, or ideally use match to cover both cases, like this:

let {JSON} = global

def safeParse(text) do
  match try JSON.parse(text)
  case ["ok", obj] then obj
  case ["fail", _] then {}
  end
end

def show(text) do
  console.log(safeParse(text))
end

show("[1, 2]")
#=> [1, 2]

show("json error")
#=> {}

Error- and throw- expressions

Squiggle has two keywords for throwing exceptions.

The most important is error. It is used like:

error "Oopsy!"

It throws an instance of JavaScript's Error with the message passed.

There is also throw which takes an exception and throws it:

match try someFunction()
case ["ok", value] then
  # use the value
case ["fail", err] then
  # do some clean up
  throw err
end

This form is not advised, except for re-throwing exceptions caught elsewhere, since the form error is much shorter.