Static Analysis and Dynamic Evaluation

As with any strongly typed programming language, WDL is processed in two distinct phases by the implementation: static analysis and dynamic evaluation.

• Static analysis is the process of parsing the WDL document and performing type inference - that is, making sure the WDL is syntactically correct, and that there is compatibility between the expected and actual types of all declarations, expressions, and calls.
• Dynamic evaluation is the process of evaluating all WDL expressions and calls at runtime, when all of the user-specified inputs are available.

An implementation should raise an error as early as possible when processing a WDL document. For example, in the following task the sub function is being called with an Int argument rather than a String. This function call cannot be evaluated successfully, so the implementation should raise an error during static analysis, rather than waiting until runtime.

task bad_sub {
  Int i = 111222333
  String s = sub(i, "2", "4")
}

On the other hand, in the following example all of the types are compatible, but if the hello.txt file does not exist when the File value is created (when evaluating the declaration File f = "hello.txt"), then an error will be raised.

task missing_file {
  File f = "hello.txt"

  command <<<
  printf "~{sep(","), read_lines(f)}"
  >>>
}