samples/iyi/config.iyi

# The first program that reads text - a `key = value` file, with the line
# number in its error.
#
#   ./bin/iyi run samples/iyi/config.iyi
#
# Text comes apart into lines, a line into its two halves around a `=`, and
# each half loses its whitespace; a line that does not parse is an error
# value carrying where it was (SPEC.md III.1), not a printed message. Like
# `basics` and `inventory`, this documents no rule: it is here because the
# prelude grows only when a program in this repository needs something
# (SPEC.md 0.1.0 item 3), and cutting text is what the third program does.
module samples/config

struct ParseError
  property line : Int32
  property message : String

  def initialize(@line : Int32, @message : String)
  end
end

impl Error for ParseError
  def message : String
    "line #{@line}: #{@message}"
  end
end

def parse(text : String) : Hash(String, String) | ParseError
  config = {} of String => String
  text.lines.each_with_index do |raw, index|
    line = raw.strip
    next if line.empty? || line.starts_with?('#')
    eq = line.index('=')
    return ParseError.new(index + 1, "no '=' in #{line.inspect}") unless eq
    key = line[0, eq].strip
    return ParseError.new(index + 1, "empty key") if key.empty?
    config[key] = line[eq + 1, line.size - eq - 1].strip
  end
  config
end

good = "# the service\nname = iyi\nport= 8080\n\nmode =fast\n"
case parse(good)
in Hash(String, String)
  puts it                  # => {"name" => "iyi", "port" => "8080", "mode" => "fast"}
  puts it["port"].to_i + 1 # => 8081
  puts it.keys.join(", ")  # => name, port, mode
in ParseError then puts "error: #{it.message}"
end

bad = "name = iyi\nbroken line\n"
case parse(bad)
in Hash(String, String) then puts "parsed"
in ParseError           then puts "error: #{it.message}" # => error: line 2: no '=' in "broken line"
end

# The pieces a parser cuts, on their own.
puts "a,,b".split(',')               # => ["a", "", "b"]
puts "one\ntwo\n".lines              # => ["one", "two"]
puts "  padded\t".strip.inspect      # => "padded"
puts "config.iyi".ends_with?(".iyi") # => true