feat: add invalid_result helper function

Closes #345

Cargo.toml

@@ -25,6 +25,7 @@ csv-core = { path = "csv-core", version = "0.1.11" }
 itoa = "1"
 ryu = "1"
 serde = "1.0.55"
+serde-value = "0.7.0"
 
 [dev-dependencies]
 bstr = { version = "1.7.0", default-features = false, features = ["alloc", "serde"] }

examples/tutorial-read-serde-invalid-03.rs

@@ -0,0 +1,30 @@
+#![allow(dead_code)]
+use std::{error::Error, io, process};
+
+use serde::Deserialize;
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "PascalCase")]
+struct Record {
+    latitude: f64,
+    longitude: f64,
+    #[serde(deserialize_with = "csv::invalid_result")]
+    population: Result<u64, String>,
+    city: String,
+    state: String,
+}
+
+fn run() -> Result<(), Box<dyn Error>> {
+    let mut rdr = csv::Reader::from_reader(io::stdin());
+    for result in rdr.deserialize() {
+        let record: Record = result?;
+        println!("{:?}", record);
+    }
+    Ok(())
+}
+
+fn main() {
+    if let Err(err) = run() {
+        println!("{}", err);
+        process::exit(1);
+    }
+}

src/lib.rs

@@ -300,6 +300,9 @@ impl Default for Trim {
 /// `Option<T>` is deserialized with non-empty but invalid data, then the value
 /// will be `None` and the error will be ignored.
 ///
+/// Use the [`invalid_result`](./fn.invalid_result.html) function if you want to
+/// return the invalid values as `Err<String>` instead of discarding them.
+///
 /// # Example
 ///
 /// This example shows how to parse CSV records with numerical data, even if
@@ -343,3 +346,97 @@ where
 {
     Option::<T>::deserialize(de).or_else(|_| Ok(None))
 }
+
+/// A custom Serde deserializer for possibly invalid `Result<T, String>` fields.
+///
+/// When deserializing CSV data, it is sometimes desirable to return separately
+/// fields with invalid data. For example, there might be a field that is
+/// usually a number, but will occasionally contain garbage data that causes
+/// number parsing to fail.
+///
+/// You might be inclined to use, say, `Result<i32, String>` for fields such at
+/// this. However this will not compile out of the box, because Serde does not
+/// know when to return `Ok<i32>` and when to return `Err<String>`.
+///
+/// This function allows you to define the following behavior: if `Result<T,
+/// String>` is deserialized with valid data, then the valid value will be
+/// returned as `Ok<T>`, while if it is deserialized with empty or invalid data,
+/// then the invalid value will be lossily converted to `String` and returned as
+/// `Err<String>`.
+///
+/// Use the [`invalid_option`](./fn.invalid_option.html) function if you want to
+/// discard the invalid values instead of returning them as `Err<String>`.
+///
+/// # Example
+///
+/// This example shows how to parse CSV records with numerical data, even if
+/// some numerical data is absent or invalid. Without the
+/// `serde(deserialize_with = "...")` annotations, this example would not
+/// compile.
+///
+/// ```
+/// use std::error::Error;
+///
+/// #[derive(Debug, serde::Deserialize, Eq, PartialEq)]
+/// struct Row {
+///     #[serde(deserialize_with = "csv::invalid_result")]
+///     a: Result<i32, String>,
+///     #[serde(deserialize_with = "csv::invalid_result")]
+///     b: Result<i32, String>,
+///     #[serde(deserialize_with = "csv::invalid_result")]
+///     c: Result<i32, String>,
+/// }
+///
+/// # fn main() { example().unwrap(); }
+/// fn example() -> Result<(), Box<dyn Error>> {
+///     let data = "\
+/// a,b,c
+/// 5,\"\",xyz
+/// ";
+///     let mut rdr = csv::Reader::from_reader(data.as_bytes());
+///     if let Some(result) = rdr.deserialize().next() {
+///         let record: Row = result?;
+///         assert_eq!(record, Row { a: Ok(5), b: Err(String::new()), c: Err(String::from("xyz")) });
+///         Ok(())
+///     } else {
+///         Err(From::from("expected at least one record but got none"))
+///     }
+/// }
+/// ```
+pub fn invalid_result<'de, D, T>(
+    de: D,
+) -> result::Result<result::Result<T, String>, D::Error>
+where
+    D: Deserializer<'de>,
+    T: Deserialize<'de>,
+{
+    let value = serde_value::Value::deserialize(de)?;
+    let result = T::deserialize(value.clone()).map_err(|_| match value {
+        serde_value::Value::Bool(b) => b.to_string(),
+        serde_value::Value::U8(u) => u.to_string(),
+        serde_value::Value::U16(u) => u.to_string(),
+        serde_value::Value::U32(u) => u.to_string(),
+        serde_value::Value::U64(u) => u.to_string(),
+        serde_value::Value::I8(i) => i.to_string(),
+        serde_value::Value::I16(i) => i.to_string(),
+        serde_value::Value::I32(i) => i.to_string(),
+        serde_value::Value::I64(i) => i.to_string(),
+        serde_value::Value::F32(f) => f.to_string(),
+        serde_value::Value::F64(f) => f.to_string(),
+        serde_value::Value::Char(c) => c.to_string(),
+        serde_value::Value::String(s) => s,
+        serde_value::Value::Unit => String::new(),
+        serde_value::Value::Option(option) => {
+            format!("{:?}", option)
+        }
+        serde_value::Value::Newtype(newtype) => {
+            format!("{:?}", newtype)
+        }
+        serde_value::Value::Seq(seq) => format!("{:?}", seq),
+        serde_value::Value::Map(map) => format!("{:?}", map),
+        serde_value::Value::Bytes(bytes) => {
+            String::from_utf8_lossy(&bytes).into_owned()
+        }
+    });
+    Ok(result)
+}

src/tutorial.rs

@@ -1136,6 +1136,62 @@ function is a generic helper function that does one very simple thing: when
 applied to `Option` fields, it will convert any deserialization error into a
 `None` value. This is useful when you need to work with messy CSV data.
 
+Sometimes you might need to return invalid fields instead of discarding them.
+For this you can use the similar
+[`invalid_result`](../fn.invalid_result.html)
+function, which works as follows: when applied to `Result<T, String>` fields,
+it will convert any invalid filed to a `String` and return it as `Err(string)`.
+Note that invalid UTF-8 bytes are lossily converted to `String`, therefore this
+function will never fail.
+
+This behavior can be achieved with very minor changes to the previous example:
+
+```no_run
+//tutorial-read-serde-invalid-03.rs
+# #![allow(dead_code)]
+# use std::{error::Error, io, process};
+#
+# use serde::Deserialize;
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "PascalCase")]
+struct Record {
+    latitude: f64,
+    longitude: f64,
+    #[serde(deserialize_with = "csv::invalid_result")]
+    population: Result<u64, String>,
+    city: String,
+    state: String,
+}
+
+fn run() -> Result<(), Box<dyn Error>> {
+    let mut rdr = csv::Reader::from_reader(io::stdin());
+    for result in rdr.deserialize() {
+        let record: Record = result?;
+        println!("{:?}", record);
+    }
+    Ok(())
+}
+#
+# fn main() {
+#     if let Err(err) = run() {
+#         println!("{}", err);
+#         process::exit(1);
+#     }
+# }
+```
+
+If you compile and run this last example, then it should run to completion just
+like the previous one but with the following output:
+
+```text
+$ cargo build
+$ ./target/debug/csvtutor < uspop-null.csv
+Record { latitude: 65.2419444, longitude: -165.2716667, population: Err(""), city: "Davidsons Landing", state: "AK" }
+Record { latitude: 60.5544444, longitude: -151.2583333, population: Ok(7610), city: "Kenai", state: "AK" }
+Record { latitude: 33.7133333, longitude: -87.3886111, population: Err(""), city: "Oakman", state: "AL" }
+# ... and more
+```
+
 # Writing CSV
 
 In this section we'll show a few examples that write CSV data. Writing CSV data