If / else

if cond then { ... } else { ... } runs one of two blocks. if cond then a else b chooses one of two values. There is no ternary operator.

if-else.lithe

(* `if` with blocks is a statement. Chain conditions with `else if`. *)
 
storage is record {
  size : uint;
}
 
external function recordSize(const x : uint) is {
  if x < 10 then {
    storage.size := 1;
  } else if x < 100 then {
    storage.size := 2;
  } else {
    storage.size := 3;
  }
}
 
(* `if` with values is an expression. Both branches are required and share a type. *)
external pure function min(const a : uint; const b : uint) : uint is
  if a < b then a else b
 
external pure function classify(const x : uint) : uint is {
  const size : uint := if x < 10 then 1 else if x < 100 then 2 else 3;
} with size

An if expression can go anywhere a value can, like a case: a function body, a with result, a const initializer, or an operand.
A branch can be fail E, as a case arm can: if amount > 0 then amount else fail Zero.
Inside each branch the compiler knows whether the condition held, so if a >= b then a - b else 0 has no underflow check.