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Problem with + and optional variable

See original GitHub issue

Hello! I used this snippet of code successfully on the Cromwell with Google backend:

task fastqc {
  input {
    File f1
    File? f2
    Int? cpu=1
    Int? machine_mem_gb
    Int? preemptible_attempts
    String mode

    Float f2size = if (mode == "PE") then size(f2, "GB") else 0.0
    Int disk_space_gb = ceil(size(f1, "GB") + f2size) + 20
  }

  command {
      fastqc -t ~{cpu} --outdir $PWD ~{f1} ~{f2}
  }

  output {
      Array[File] html = glob("*html")
      Array[File] zip  = glob("*zip")
  }

  runtime {
    docker: "biocontainers/fastqc:v0.11.5_cv2"
    memory: select_first([machine_mem_gb, 4]) + " GB"
    cpu: cpu
    disks: "local-disk " + disk_space_gb + " HDD"
    preemptible: select_first([preemptible_attempts, 3])
  }
}

On local backend I got this error: _Failed to evaluate input '_disk_space_gb’ (reason 1 of 1): Sorry! Operation + is not supported on empty optional values. You might resolve this using select_first([optional, default]) to guarantee that you have a filled value.

I tried following the advice (even though I don’t see what is missing) Float f2size = select_first([if (mode == “PE”) then size(f2, “GB”) else 0.0]) And then I got an even more confusing error: Failed to process task definition ‘fastqc’ (reason 1 of 1): Failed to process expression ‘select_first([f2size, select_first([if (mode == “PE”) then size(f2, “GB”) else 0.0])])’ (reason 1 of 1): Invalid parameter ‘ArrayLiteral(Vector(TernaryIf(Equals(IdentifierLookup(mode),StringLiteral(PE)),Size(IdentifierLookup(f2),Some(StringLiteral(GB))),PrimitiveLiteralExpressionElement(WomFloat(0.0)))))’. Expected an array of optional values (eg ‘Array[X?]’) but got ‘Array[Float]+’

Can you help me understand? It all passes Womtools validation.