Undefined behavior for string to file coercion when file does not exist

[stxue1]

It's unclear if a string being coerced to a file should error or not depending on if the file exists and that the file is not outputted in the WDL.

For example:

version 1.1
workflow test_map {
  File f = "path/to/file"

  output {
  }
}

It's unclear whether this workflow should error on File f if the file does not exist.

MiniWDL seems to not error on these cases, but only errors if the file is to be outputted, for example:

version 1.1

workflow test_map {
  File f = "path/to/file"

  output {
    File out = f
  }
}

A lot of the SPEC tests also depend on this behavior, for example:

wdl/SPEC.md

Lines 1190 to 1208 in 664adc3

	Example: test_map.wdl
	```wdl
	version 1.2
	workflow test_map {
	Map[Int, Int] int_to_int = {1: 10, 2: 11}
	Map[String, Int] string_to_int = { "a": 1, "b": 2 }
	Map[File, Array[Int]] file_to_ints = {
	"/path/to/file1": [0, 1, 2],
	"/path/to/file2": [9, 8, 7]
	}
	output {
	Int ten = int_to_int[1] # evaluates to 10
	Int b = string_to_int["b"] # evaluates to 2
	Array[Int] ints = file_to_ints["/path/to/file1"] # evaluates to [0, 1, 2]
	}
	}

In particular, this section about type coercion talks about string to file coercion, but doesn't mention whether the file must exist at the coercion step.

Should execution engines ensure that files must exist at coercion or only at a workflow/task output?

[patmagee]

As of now there is no requirement in the spec for a file declaration to actually exist. You can declare any string as a file, it will simply fail when the engine tries to localize it or read it.

This behaviour is consistent with other programming languages which allow files to be declared without enforcing their existence

[jdidion]

Agree w @patmagee. The question is whether we want to make this explicit in the spec. I think we should.