feat(did-you-mean): for subcommands

If an argument is not understood as subcommand, but has a
high-confidence match in the list of all known subcommands, we will use
this one to print a customized error message.

Previously, it would say that a positional argument wasn't understood,
now it will say that a subcommand was unknown, and if the user meant
`high-confidence-candidate`.

If the argument doesn't sufficiently match any subcommand, the default
handling will take over and try to treat it as positional argument.

* added dependency to `strsym` crate
* new `did_you_mean` function uses `strsim::jaro_winkler(...)` to look
  for good candidates.

Related to #103

Cargo.toml

@@ -13,6 +13,9 @@ keywords = ["argument", "command", "arg", "parser", "parse"]
 
 license = "MIT"
 
+[dependencies]
+strsim = "*"
+
 [features]
 default=[]
 

clap-tests/run_tests.py

@@ -35,6 +35,11 @@ SUBCOMMANDS:
     help      Prints this message
     subcmd    tests subcommands'''
 
+_sc_dym_usage = '''Subcommand "subcm" is unknown. Did you mean "subcmd" ?
+USAGE:
+\tclaptests [POSITIONAL] [FLAGS] [OPTIONS] [SUBCOMMANDS]
+For more information try --help'''
+
 _excluded = '''The argument '--flag' cannot be used with '-F'
 USAGE:
 \tclaptests [positional2] -F --long-option-2 <option2>
@@ -220,6 +225,7 @@ cmds = {'help short:         ': ['{} -h'.format(_bin), _help],
 		'F(s),O(s),P:        ': ['{} value -f -o some'.format(_bin), _fop],
 		'F(l),O(l),P:        ': ['{} value --flag --option some'.format(_bin), _fop],
 		'F(l),O(l=),P:       ': ['{} value --flag --option=some'.format(_bin), _fop],
+        'sc dym:             ': ['{} subcm'.format(_bin), _sc_dym_usage],
 		'sc help short:      ': ['{} subcmd -h'.format(_bin), _schelp],
 		'sc help long:       ': ['{} subcmd --help'.format(_bin), _schelp],
 		'scF(l),O(l),P:      ': ['{} subcmd value --flag --option some'.format(_bin), _scfop],

src/app.rs

@@ -11,6 +11,30 @@ use args::{ ArgMatches, Arg, SubCommand, MatchedArg};
 use args::{ FlagBuilder, OptBuilder, PosBuilder};
 use args::ArgGroup;
 
+use strsim;
+
+/// Produces a string from a given list of possible values which is similar to 
+/// the passed in value `v` with a certain confidence.
+/// Thus in a list of possible values like ["foo", "bar"], the value "fop" will yield
+/// `Some("foo")`, whereas "blark" would yield `None`.
+fn did_you_mean<'a, I, T>(v: &str, possible_values: I) -> Option<&'a str> 
+    where       T: AsRef<str> + 'a,
+                I: IntoIterator<Item=&'a T>{
+
+    let mut candidate: Option<(f64, &str)> = None;
+    for pv in possible_values.into_iter() {
+        let confidence = strsim::jaro_winkler(v, pv.as_ref());
+        if confidence > 0.8 && (candidate.is_none() || 
+                               (candidate.as_ref().unwrap().0 < confidence)) {
+            candidate = Some((confidence, pv.as_ref()));
+        }
+    }
+    match candidate {
+        None => None,
+        Some((_, candidate)) => Some(candidate),
+    }
+}
+
 /// Used to create a representation of a command line program and all possible command line
 /// arguments for parsing at runtime.
 ///
@@ -1296,6 +1320,16 @@ impl<'a, 'v, 'ab, 'u, 'h, 'ar> App<'a, 'v, 'ab, 'u, 'h, 'ar>{
                     break;
                 }
 
+                if let Some(candidate_subcommand) = did_you_mean(&arg, self.subcommands.keys()) {
+                    self.report_error(
+                        format!("Subcommand \"{}\" is unknown. Did you mean \"{}\" ?",
+                            arg,
+                            candidate_subcommand),
+                        true,
+                        true,
+                        None);
+                }
+
                 if self.positionals_idx.is_empty() {
                     self.report_error(
                         format!("Found argument \"{}\", but {} wasn't expecting any",

src/lib.rs

@@ -348,6 +348,7 @@
 //!   - `Arg::new()` -> `Arg::with_name()`
 //!   - `Arg::mutually_excludes()` -> `Arg::conflicts_with()`
 //!   - `Arg::mutually_excludes_all()` -> `Arg::conflicts_with_all()`
+extern crate strsim;
 
 pub use args::{Arg, SubCommand, ArgMatches, ArgGroup};
 pub use app::App;