Opening a fas format file on Biostrings package in R - r

Question

I am trying to work on a fas extension file with using 'Biostrings' package. I have attempted a multiple different ways, exhausted google searches without much success on different webs/blogs/video tutorials.
Please consider following file path:
"/mydesktop/DNAfile.fas"
I have successfully installed Biostrings package and XVector.
library(Biostrings)
library(Vector)
Then I wrote below lines:
fas1 <- system.file("extdata", "/mydesktop/DNAfile.fas", package="Biostrings")
dna <- readDNAStringSet(fas1)
However, this created error as this:
Error in .Call2("new_input_filexp", filepath, PACKAGE = "XVector") : cannot open file ''
Can someone guide me through this? Someone advised some other user on some other blog that he should consider changing fas into FASTA, but how would that change the workflow?
Thank you!

Answer 1

I probably found an answer. Not quite sure if I can answer my own question. First, both comments by #Maurits and #Poshi were right. I appreciate them again. It's probably a longer version of what they wisely mentioned above, but my thought process is below. This is for other beginners who might fall into same misunderstanding that I went through... to save their time and energy.
First, the fas (or fasta, fa) files are essentially the same, if the file creator did not intend to fool the user by putting a different extension on purpose. One way to know whether it truly is based on the same format is, just by opening them in any ASCII text editor or equivalent environment and browse the first few lines. However, beware if the file size is huge, it will consume your RAM and make your computer uncomfortable.
Second, I checked whether the file truly contains the definition line as well as the sequence. I know it's a blunt way (and may not be totally accurate), but even before I went to ASCII level, I opened it on R with below:
g <- gzcon(file("/MyDesktop/Ecoli_Genome.fas", "r")) # set to your path
fas <- readLines(g) # read directly from .gz file
close(g) # close the file connection
head(fas) # lines of the FASTQ formatted file
Here you can check all sequences with your own eyes.
But obviously, this does not make you compatible to use the Biostrings on this sequence!
Make sure you correctly installed Biostrings. It cannot be installed via install.packages("Biostrings"). Here is the source.
source("https://bioconductor.org/biocLite.R")
The webpage is:
https://bioconductor.org/packages/release/bioc/html/Biostrings.html
Third, understand the fas (or fasta) can store all sequence in one line. It's basically a complex form of a list, and your sequence can be stored in different ways, but contiguous bases usually need not to be stored fragmented. So my sequence was all in one line!
So the solution to my question was:
library(Biostrings)
dna <- readDNAStringSet("/MyDesktop/Ecoli_Genome.fas")
You can confirm by opening it:
> dna
A DNAStringSet instance of length 1
width seq names
[1] 4641652 AGCTTTTCATTCTGACTGCAACGGGCA...AAAAAACGCCTTAGTAAGTATTTTTC U00096.3
Let me know any further comments to learn. Thank you