feat(pickable): introduce SinglePickable trait

Extract a simplified trait for types that consume exactly one
argument, with a blanket implementation of the existing Pickable trait

2 files changed, 51 insertions, 15 deletions

diff --git a/mingling_picker/src/builtin/pick_string.rs b/mingling_picker/src/builtin/pick_string.rs
index 4a65873..c96f667 100644
--- a/mingling_picker/src/builtin/pick_string.rs
+++ b/mingling_picker/src/builtin/pick_string.rs
@@ -1,19 +1,11 @@
-use crate::parselib::{SingleMatcher, seek_single};
-use crate::pickable_needed::*;
+use crate::PickerArgResult::NotFound;
+use crate::{SinglePickable, pickable_needed::*};
 
-impl<'a> Pickable<'a> for String {
-    fn get_attr(flag: &'a PickerArg<'a, Self>) -> PickerArgAttr {
-        PickerArgAttr::positional_or_single(flag)
-    }
-
-    fn tag(ctx: TagPhaseContext) -> Vec<usize> {
-        SingleMatcher::tag(ctx)
-    }
-
-    fn pick(raw_strs: &[&str]) -> PickerArgResult<Self> {
-        match seek_single(raw_strs) {
-            Some(v) => PickerArgResult::Parsed(v.to_string()),
-            None => PickerArgResult::NotFound,
+impl SinglePickable for String {
+    fn pick_single(str: Option<&str>) -> PickerArgResult<Self> {
+        match str {
+            Some(str) => PickerArgResult::Parsed(str.to_string()),
+            None => NotFound,
         }
     }
 }
diff --git a/mingling_picker/src/pickable.rs b/mingling_picker/src/pickable.rs
index c88e49b..7424e6f 100644
--- a/mingling_picker/src/pickable.rs
+++ b/mingling_picker/src/pickable.rs
@@ -83,3 +83,47 @@ pub struct TagPhaseContext<'a> {
     /// has already been tagged by another `Pickable`.
     pub mask: &'a [u8],
 }
+
+/// `SinglePickable` trait defines how to parse a type from a single command-line argument.
+///
+/// This trait provides a simplified interface for types that consume exactly one argument value.
+/// It is automatically implemented by the blanket `impl` of [`Pickable`], so types implementing
+/// `SinglePickable` will work with the full `Pickable` argument parsing system.
+///
+/// # Type Parameters
+///
+/// * `Self` - The type to be parsed from a single argument string.
+pub trait SinglePickable
+where
+    Self: Sized,
+{
+    /// Parse a single optional string value into an instance of `Self`.
+    ///
+    /// # Parameters
+    ///
+    /// * `str` - An `Option<&str>` representing the raw argument value. If `None`,
+    ///   it indicates that no argument value was provided (e.g., for flag-like arguments).
+    ///
+    /// # Returns
+    ///
+    /// Returns [`PickerArgResult<Self>`], i.e., the parsed `Self` instance on success,
+    /// or an appropriate error message on failure.
+    fn pick_single(str: Option<&str>) -> PickerArgResult<Self>;
+}
+
+impl<'a, S> Pickable<'a> for S
+where
+    S: SinglePickable,
+{
+    fn get_attr(flag: &'a PickerArg<'a, Self>) -> PickerArgAttr {
+        PickerArgAttr::positional_or_single(flag)
+    }
+
+    fn tag(ctx: TagPhaseContext) -> Vec<usize> {
+        crate::parselib::SingleMatcher::tag(ctx)
+    }
+
+    fn pick(raw_strs: &[&str]) -> PickerArgResult<Self> {
+        Self::pick_single(crate::parselib::seek_single(raw_strs))
+    }
+}