rust 41 lines · 7 steps

Custom CSV deserialization with serde in Rust

Parsing a CSV of transactions into typed structs, with field renaming, defaults, and a custom money parser.

Explained by highlit
Intermediate serde csv deserialization error-handling custom-parser
1use std::error::Error;
2use std::path::Path;
3 
4use serde::Deserialize;
5 
6#[derive(Debug, Deserialize)]
7struct Transaction {
8 id: u64,
9 #[serde(rename = "date")]
10 booked_at: String,
11 description: String,
12 #[serde(deserialize_with = "parse_cents")]
13 amount: i64,
14 #[serde(default)]
15 reconciled: bool,
16}
17 
18fn parse_cents<'de, D>(deserializer: D) -> Result<i64, D::Error>
19where
20 D: serde::Deserializer<'de>,
21{
22 let raw = String::deserialize(deserializer)?;
23 let cleaned = raw.trim().replace(['$', ','], "");
24 let dollars: f64 = cleaned.parse().map_err(serde::de::Error::custom)?;
25 Ok((dollars * 100.0).round() as i64)
26}
27 
28fn load_transactions(path: impl AsRef<Path>) -> Result<Vec<Transaction>, Box<dyn Error>> {
29 let mut reader = csv::ReaderBuilder::new()
30 .trim(csv::Trim::All)
31 .flexible(false)
32 .from_path(path)?;
33 
34 let mut transactions = Vec::new();
35 for result in reader.deserialize() {
36 let record: Transaction = result?;
37 transactions.push(record);
38 }
39 
40 Ok(transactions)
41}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1serde attributes let you map messy external data onto clean, typed fields without hand-written parsing loops.
  2. 2A deserialize_with function turns any string into a domain value while still routing failures through serde's error channel.
  3. 3Storing money as integer cents sidesteps the rounding pitfalls of holding currency in floating point.

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