r - Nextflow：将脚本变量作为字符串输入 R？

Question

我正在尝试学习 Nextflow，并努力让这个简单的脚本在 R 中工作：

echo true

col=Channel.from(2,4)

process getCols {

  input:
   val WD from "data/"
   val col

  script:
  """
  #!/usr/bin/env Rscript

  dat=read.csv(list.files(as.character($WD))
  dat[,$col]
  """

}

这将返回以下错误：

Error executing process > 'getCols (1)'

Caused by:
  Process `getCols (1)` terminated with an error exit status (1)

Command executed:

  #!/usr/bin/env Rscript
  
  dat=read.csv(list.files(as.character(data/))
  dat[,2]

Command exit status:
  1

Command output:
  (empty)

Command error:
  Error: unexpected ')' in "dat=read.csv(list.files(as.character(data/)"
  Execution halted

我认为问题在于 $WD 没有被解释为字符串。我怎样才能解决这个问题？