Line Filters in Crystal

Our line filter program reads input from stdin, processes it, and then prints a derived result to stdout. In this case, we’ll create a program that capitalizes all input text. This pattern can be used to create your own line filters in Crystal.

# Import required modules
require "readline"

# Main program
while line = Readline.readline
  # Convert the line to uppercase
  uppercase_line = line.upcase

  # Print the uppercased line
  puts uppercase_line
end

# Check for errors during input
if (err = Readline.error)
  STDERR.puts "error: #{err}"
  exit(1)
end

In this Crystal version:

1. We use the Readline module to read input line by line.

2. The while loop continues reading lines until there’s no more input.

3. Each line is converted to uppercase using the upcase method.

4. The uppercased line is then printed using puts.

5. After the loop, we check for any errors that might have occurred during input.

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 | crystal line_filter.cr
HELLO
FILTER

This Crystal implementation provides the same functionality as the original Go program, reading input line by line, converting each line to uppercase, and printing the result.

Crystal’s standard library provides convenient methods for string manipulation and I/O operations, making it straightforward to implement this kind of text processing task.