A command-line argument is a string supplied when a program starts. Command-line tools use arguments to choose input and configure behavior. In Haskell, `System.Environment` separates reading this external input from the pure code that transforms it. section 3.3 section 3.2
`getArgs :: IO [String]` reads all supplied arguments as a list of strings. Reading them is an `IO` action because it interacts with the operating system; parsing the resulting list can remain pure. Keeping that boundary small lets parsing, formatting, filtering, and numbering be ordinary functions. section 3.3.1 section 3.2.2
Accepting one filename
A minimal line-numbering command can use the form `nl <filename>`. Pattern matching recognizes exactly one argument with `[filePath]`; a fallback pattern handles missing or extra arguments, such as by showing help text. `FilePath` is a synonym for `String`, but communicates the intended role of the value. section 3.3.1
Listing 3.4 Simple argument-parsing function
parseArguments :: [String] -> FilePath parseArguments [filePath] = filePath #1 parseArguments _ = undefined #2
This parser shows the smallest useful split: extract a single filename from a one-element list, and handle every other argument shape separately. section 3.3.1
As a command grows, arguments can include a filename plus options. One approach fixes the order: options first and the filename last. Reversing the argument list makes the filename easy to take with `filename : options`; each remaining string is parsed into a domain value, while unrecognized options become `Nothing` and are ignored by `mapMaybe`. section 4.4.1
Options can be represented explicitly rather than passed around as unchecked strings:
- Use an algebraic data type for the supported choices, such as reverse numbering, skipping empty lines, and left alignment. section 4.4.1
- Convert each recognized command-line string to `Just` an option value; return `Nothing` for an unrecognized string. section 4.4.1
- Select behavior from parsed options, for example choosing a numbering function when `SkipEmptyLines` is present. section 4.4.1
Listing 4.14 Function to parse a list of command line arguments
import Data.Maybe
...
parseArguments :: [String] -> (Maybe FilePath, [LineNumberOption])
parseArguments args = case reverse args of #1
[] -> (Nothing, []) #2
(filename : options) ->
( Just filename, #3
mapMaybe lnOptionFromString options #4
)
The complete list parser extracts an optional filename and turns recognized option strings into typed values. section 4.4.1
For larger applications, argument parsing can be provided through `optparse-applicative`. This fits an application structure that also uses a read-only environment, event log, state, file operations, and exception handling. section 15.0
