Line Filters in Chapel

A line filter is a common type of program that reads input on stdin, processes it, and then prints some derived result to stdout. grep and sed are common line filters.

Here’s an example line filter in Chapel that writes a capitalized version of all input text. You can use this pattern to write your own Chapel line filters.

use IO;
use String;

proc main() {
    // Read lines from stdin
    var line: string;
    while readLine(line) {
        // Convert the line to uppercase
        var ucl = line.toUpper();
        
        // Write out the uppercased line
        writeln(ucl);
    }

    // Check for errors during reading
    if (stdin.error()) {
        stderr.writeln("error: ", stdin.errorMsg());
        exit(1);
    }
}

In this Chapel implementation:

1. We use the IO and String modules for input/output operations and string manipulation.

2. The main procedure is the entry point of our program.

3. We use a while loop with readLine() to read lines from stdin. This is similar to the scanner in the original example.

4. For each line, we convert it to uppercase using the toUpper() method.

5. We then write the uppercased line to stdout using writeln().

6. After the loop, we check for any errors that occurred during reading. If there was an error, we print it to stderr and exit with a non-zero status code.

To try out our line filter, first make a file with a few lowercase lines.

$ echo 'hello'   > /tmp/lines
$ echo 'filter' >> /tmp/lines

Then use the line filter to get uppercase lines.

$ cat /tmp/lines | ./line-filters
HELLO
FILTER

Note: In Chapel, you would first need to compile the program (let’s say it’s saved as line-filters.chpl) using the Chapel compiler (chpl), and then run the resulting executable:

$ chpl line-filters.chpl
$ cat /tmp/lines | ./line-filters

This Chapel implementation provides the same functionality as the original example, reading lines from stdin, converting them to uppercase, and writing them to stdout.