在使用biomaRt库的函数上下文中调用变量时出现意外结果

我有一个一般性的问题，我认为可以归结为某种范围问题

下面是使用biomaRt的getSequence（）函数的公式片段。用户在自定义函数中输入（1）基因名称，并可选地输入（2）上游要导入的碱基对数

# Load libraries
library(biomaRt)
# Let's make a custom "getSequence" function
getUpstream <- function(x, bp.upstream = 50){
    bp.upstream <- bp.upstream
    ensembl <- useMart("ensembl", dataset = "hsapiens_gene_ensembl")
    upstream.master <- NULL
    for(i in x){
        upstream.i <- getSequence(id = i,
            type = "hgnc_symbol",
            seqType = "coding_gene_flank",
            upstream = bp.upstream,
            mart = ensembl
        )
        upstream.master <- rbind(upstream.master, upstream.i)
    }
    return(upstream.master)
}

意外的是，如果没有以下行，函数将无法工作：

bp.upstream <- bp.upstream

---

bp.upstream这里有一个避免范围问题的解决方法

# Load libraries
library(biomaRt)
# Let's make a custom "getSequence" function
 getUpstream <- function(x, bp.upstream = 50){
  ensembl <- useMart("ensembl", dataset = "hsapiens_gene_ensembl")
  upstream.master <- lapply(x, function(i,stream)
                         getSequence(id = i,
                              type = "hgnc_symbol",
                              seqType = "coding_gene_flank",
                              upstream = stream,
                              mart = ensembl),stream=bp.upstream)


  upstream.master
}

#加载库
图书馆（生物艺术）
#让我们创建一个自定义的“getSequence”函数
逆流而上
# Load libraries
library(biomaRt)
# Let's make a custom "getSequence" function
 getUpstream <- function(x, bp.upstream = 50){
  ensembl <- useMart("ensembl", dataset = "hsapiens_gene_ensembl")
  upstream.master <- lapply(x, function(i,stream)
                         getSequence(id = i,
                              type = "hgnc_symbol",
                              seqType = "coding_gene_flank",
                              upstream = stream,
                              mart = ensembl),stream=bp.upstream)


  upstream.master
}