干净的语言：在文件末尾附加数字，fwritei 不起作用

Question

I'm trying to write function that receive [String] which are names of files, String which is the name of the files directory and *f . The function will append to each file an integer in the end.

Here is what I got so far:

import StdEnv
import StdFile
import FileManipulation

appendNumInEndOfVmFiles :: [String] String *f -> String
appendNumInEndOfVmFiles [] dirname w = "finished"
appendNumInEndOfVmFiles [x:xs] dirname w
# path = dirname +++ "\\\\" +++ x
# (ok,file,files) = fopen path FAppendText w
# file = fwritei 12 file 
# (ok2,_) = fclose file w
= appendNumInEndOfVmFiles xs dirname w


Start w
// 1. Receive name of directory from the user.
# (io,w) = stdio w                                  // open stdio
# io = fwrites "Enter name of directory:\n" io      // ask for name
# (name,io) = freadline io                          // read in name
# name = name % (0, size name - 2)                  // remove \n from name
# (ok,w) = fclose io w                              // close stdio
| not ok = abort "Couldn't close stdio"             // abort in case of         failure

// 2. Get a list of all file names in that directory.
# (dir,w) = getDirectoryContents (RelativePath [PathDown name]) w
# fileList = getNamesOfFilesInDirectory (getEntriesList dir)

= appendNumInEndOfVmFiles (getVmFiles fileList) name w

Assume that getVmFiles is defined in my FileManipulation.dcl file and in the context of this problem name is "myDir" and file list is ["hello.vm","Wiki.vm"]

For some reason, even that I got "finished" message on the screen, the files aren't modified. No matter what kind of integer I give to fopen , even if its FWriteText or FWriteData its still doing nothing... also even if I'm using fwritec or fwrites with characters nothing happened.

What I'm missing here? Thanks a lot!

Answer 1

For some reason, even that I got "finished" message on the screen, the files aren't modified.

This is due to lazy evaluation . In appendNumInEndOfVmFiles , the result of fclose is not used, so fclose is not evaluated. Because of this, fwritei does not need to be evaluated either. You can fix this by adding a guard on ok2 :

# (ok2,_) = fclose file w
| not ok2 = abort "fclose failed\n"
= appendNumInEndOfVmFiles xs dirname w

However, the typical way to do this would be to rewrite the function to return a *f instead of a String , so that this unique value is not lost. As long as the result is used, then, the fwritei is evaluated. You can potentially make the *f argument strict (ie add a ! in front). This would make sure that it is evaluated before entering the function, so that all lingering file closes have been performed.

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There are some more issues with your code:

1. Here, w is used twice, which is illegal because it is of a strict type. You should use (ok2,w) in the guard to continue with the same environment.

    # (ok2,_) = fclose file w = appendNumInEndOfVmFiles xs dirname w

2. The appendNumInEndOfVmFiles needs to have a type context | FileSystem f | FileSystem f to resolve overloading of fopen and fclose .

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Lastly:

... even if its FWriteText or FWriteData ...

Just so you know: the difference would be that the first would write the integer in an ASCII representation whereas the second would write it binary as 4 or 8 bytes (depending on the bitwidth of your system).