From 84231621595635599edd323824e9eeb0c1fd1465 Mon Sep 17 00:00:00 2001
From: Joshua Isika <joshuaiska@gmail.com>
Date: Thu, 6 Aug 2026 13:50:38 +0300
Subject: [PATCH] decimal: replace try_from with FromStr impl

Implements exercism/rust#1037. Replaces the custom Decimal::try_from
method with `impl FromStr for Decimal`, so students (and test/example
code) parse with `.parse()` instead of a bespoke constructor.

Updates the example solution, student stub, and test file accordingly.
All 44 exercise tests and clippy pass against the updated example.
---
 exercises/practice/decimal/.meta/example.rs | 82 ++++++++++++---------
 exercises/practice/decimal/src/lib.rs       |  9 ++-
 exercises/practice/decimal/tests/decimal.rs |  4 +-
 3 files changed, 56 insertions(+), 39 deletions(-)

diff --git a/exercises/practice/decimal/.meta/example.rs b/exercises/practice/decimal/.meta/example.rs
index a11f60f8f16..fea873ccb38 100644
--- a/exercises/practice/decimal/.meta/example.rs
+++ b/exercises/practice/decimal/.meta/example.rs
@@ -1,6 +1,7 @@
 use std::cmp::Ordering;
 use std::fmt;
 use std::ops::{Add, Mul, Sub};
+use std::str::FromStr;
 
 extern crate num_bigint;
 use num_bigint::BigInt;
@@ -24,39 +25,6 @@ impl Decimal {
         value
     }
 
-    pub fn try_from(mut input: &str) -> Option<Decimal> {
-        // clear extraneous whitespace
-        input = input.trim();
-
-        // don't bother to trim extraneous zeroes
-        // leave it to users to manage their own memory
-
-        // now build a representation of the number to parse
-        let mut digits = String::with_capacity(input.len());
-        let mut decimal_index = None;
-        for ch in input.chars() {
-            match ch {
-                '0'..='9' | '-' | '+' => {
-                    digits.push(ch);
-                    if let Some(idx) = decimal_index.as_mut() {
-                        *idx += 1;
-                    }
-                }
-                '.' => {
-                    if decimal_index.is_some() {
-                        return None;
-                    }
-                    decimal_index = Some(0)
-                }
-                _ => return None,
-            }
-        }
-        Some(Decimal::new(
-            digits.parse().ok()?,
-            decimal_index.unwrap_or_default(),
-        ))
-    }
-
     /// Add precision to the less-precise value until precisions match
     ///
     /// Precision, in this case, is defined as the decimal index.
@@ -94,6 +62,47 @@ impl Decimal {
     }
 }
 
+/// Indicates that a string could not be parsed as a `Decimal`
+#[derive(Debug, PartialEq, Eq)]
+pub struct ParseDecimalError;
+
+impl FromStr for Decimal {
+    type Err = ParseDecimalError;
+
+    fn from_str(input: &str) -> Result<Self, Self::Err> {
+        // clear extraneous whitespace
+        let input = input.trim();
+
+        // don't bother to trim extraneous zeroes
+        // leave it to users to manage their own memory
+
+        // now build a representation of the number to parse
+        let mut digits = String::with_capacity(input.len());
+        let mut decimal_index = None;
+        for ch in input.chars() {
+            match ch {
+                '0'..='9' | '-' | '+' => {
+                    digits.push(ch);
+                    if let Some(idx) = decimal_index.as_mut() {
+                        *idx += 1;
+                    }
+                }
+                '.' => {
+                    if decimal_index.is_some() {
+                        return Err(ParseDecimalError);
+                    }
+                    decimal_index = Some(0)
+                }
+                _ => return Err(ParseDecimalError),
+            }
+        }
+        Ok(Decimal::new(
+            digits.parse().map_err(|_| ParseDecimalError)?,
+            decimal_index.unwrap_or_default(),
+        ))
+    }
+}
+
 macro_rules! auto_impl_decimal_ops {
     ($(#[$attr:meta])* $trait:ident, $func_name:ident, $digits_operation:expr, $index_operation:expr) => {
         impl $trait for Decimal {
@@ -179,11 +188,14 @@ mod tests {
             println!(
                 "Decimal representation of \"{}\": {}",
                 test_str,
-                Decimal::try_from(test_str).expect("This should always become a decimal")
+                test_str
+                    .parse::<Decimal>()
+                    .expect("This should always become a decimal")
             );
             assert_eq!(
                 test_str,
-                Decimal::try_from(test_str)
+                test_str
+                    .parse::<Decimal>()
                     .expect("This should always become a decimal")
                     .to_string()
             )
diff --git a/exercises/practice/decimal/src/lib.rs b/exercises/practice/decimal/src/lib.rs
index c9ad7098f33..8c6a0db404e 100644
--- a/exercises/practice/decimal/src/lib.rs
+++ b/exercises/practice/decimal/src/lib.rs
@@ -1,10 +1,15 @@
+use std::str::FromStr;
+
 /// Type implementing arbitrary-precision decimal arithmetic
 pub struct Decimal {
     // implement your type here
 }
 
-impl Decimal {
-    pub fn try_from(input: &str) -> Option<Decimal> {
+impl FromStr for Decimal {
+    // implement your error type here
+    type Err = String;
+
+    fn from_str(input: &str) -> Result<Self, Self::Err> {
         todo!("Create a new decimal with a value of {input}")
     }
 }
diff --git a/exercises/practice/decimal/tests/decimal.rs b/exercises/practice/decimal/tests/decimal.rs
index ebdf9d7d509..dc7cf5a517e 100644
--- a/exercises/practice/decimal/tests/decimal.rs
+++ b/exercises/practice/decimal/tests/decimal.rs
@@ -4,7 +4,7 @@ use decimal::Decimal;
 ///
 /// Use only when you _know_ that your value is valid.
 fn decimal(input: &str) -> Decimal {
-    Decimal::try_from(input).expect("That was supposed to be a valid value")
+    input.parse().expect("That was supposed to be a valid value")
 }
 
 /// Some big and precise values we can use for testing. [0] + [1] == [2]
@@ -185,7 +185,7 @@ fn gt_varying_negative_precisions() {
 #[test]
 #[ignore]
 fn negatives() {
-    assert!(Decimal::try_from("-1").is_some());
+    assert!("-1".parse::<Decimal>().is_ok());
     assert_eq!(decimal("0") - decimal("1"), decimal("-1"));
     assert_eq!(decimal("5.5") + decimal("-6.5"), decimal("-1"));
 }