Control Flow

If / Elif / Else

Purus uses indentation to define blocks — no curly braces {} needed.

if x lt 0
  console.log[///negative///]
elif x eq 0
  console.log[///zero///]
else
  console.log[///positive///]

if (x < 0) {
  console.log("negative");
} else if (x === 0) {
  console.log("zero");
} else {
  console.log("positive");
}

Tip

elif is an alias for else if. Both work identically:

if x lt 0
  console.log[///negative///]
else if x eq 0
  console.log[///zero///]
else
  console.log[///positive///]

This compiles to the same JavaScript as elif.

Multi-line blocks

Any statements indented under if, elif, or else belong to that block:

if logged-in
  const user be get-user[]
  console.log[///Welcome, [user.name]///]
  show-dashboard[]
else
  console.log[///Please log in///]
  redirect[///login///]

if (logged_in) {
  const user = get_user();
  console.log(`Welcome, ${user.name}`);
  show_dashboard();
} else {
  console.log("Please log in");
  redirect("login");
}

Unless

Negated if:

unless done
  keep-going[]

if (!(done)) {
  keep_going();
}

Inline if (ternary)

const result be if condition then 1 else 2

const result = condition ? 1 : 2;

Postfix modifiers

Write short conditionals and loops on a single line:

console.log[///debug///] if verbose
console.log[///skip///] unless done
console.log[item] for item in list

if (verbose) {
  console.log("debug");
}
if (!(done)) {
  console.log("skip");
}
for (const item of list) {
  console.log(item);
}

While / Until

while i lt 10
  i be i add 1

until finished
  do-work[]

while (i < 10) {
  i = i + 1;
}
while (!(finished)) {
  do_work();
}

For-in

for item in items
  console.log[item]

for (const item of items) {
  console.log(item);
}

With index:

for i; item in items
  console.log[i; item]

for (let [i, item] of items.entries()) {
  console.log(i, item);
}

For-range

for i in range 0; 10
  console.log[i]

for (let i = 0; i < 10; i++) {
  console.log(i);
}

Switch / Case / Default

switch x
  case 1 then ///one///
  case 2 then ///two///
  default ///other///

if (x === 1) {
  "one";
} else if (x === 2) {
  "two";
} else {
  "other";
}

With guards:

In case n if ..., the variable n is bound to the switch subject (value), allowing the guard condition to reference it.

switch value
  case n if n gt 0
    console.log[///positive///]
  default
    console.log[///non-positive///]

{
  const n = value;
  if (n > 0) {
    console.log("positive");
  } else {
    console.log("non-positive");
  }
}

Match / When

Deprecated

match / when is deprecated. Use switch / case / default instead.

match x
  when 1 then ///one///
  when 2 then ///two///
  else ///other///

if (x === 1) {
  "one";
} else if (x === 2) {
  "two";
} else {
  "other";
}

With guards:

In when n if ..., the variable n is bound to the match subject (value), allowing the guard condition to reference it.

match value
  when n if n gt 0
    console.log[///positive///]
  else
    console.log[///non-positive///]

{
  const n = value;
  if (n > 0) {
    console.log("positive");
  } else {
    console.log("non-positive");
  }
}

try / catch / finally

try
  risky[]
catch e
  console.log[e]
finally
  cleanup[]

try {
  risky();
} catch (e) {
  console.log(e);
} finally {
  cleanup();
}

Try as expression

const result be try
  risky[]
catch e
  default-value

let result;
try {
  result = risky();
} catch (e) {
  result = defaultValue;
}

Throw

throw new Error[///something went wrong///]
throw err if condition

throw new Error("something went wrong");
if (condition) {
  throw err;
}