Expressions

An expression is a compound statement that consists of literal values, identifiers (references to declarations or call outputs), built-in operators (e.g., + or >=), and calls to standard library functions.

A "literal" expression is one that consists only of a literal value. For example, "foo" is a literal String expression and [1, 2, 3] is a literal Array[Int] expression.

A "simple" expression is one that can be evaluated unambiguously without any knowledge of the runtime context. Literal expressions, operations on literals (e.g., 1 + 2), and function calls with literal arguments (excluding any functions that read or create Files) are all simple expressions. A simple expression cannot refer to any declarations (i.e., it cannot contain identifiers). An execution engine may choose to replace a simple expression with its literal value during static analysis.

Example: expressions_task.wdl

version 1.2

task expressions {
  input {
    Int x
  }

  command <<<
  printf "hello" > hello.txt
  >>>

  output {
    # simple expressions
    Float f = 1 + 2.2
    Boolean b = if 1 > 2 then true else false
    Map[String, Int] m = as_map(zip(["a", "b", "c"], [1, 2, 3]))

    # non-simple expressions
    Int i = x + 3  # requires knowing the value of x
    # requires reading a file that might only exist at runtime
    String s = read_string("hello.txt")
  }
}

{
  "expressions.x": 5
}

{
  "expressions.f": 3.2,
  "expressions.b": false,
  "expressions.m": {
    "a": 1,
    "b": 2,
    "c": 3
  },
  "expressions.i": 8,
  "expressions.s": "hello"
}