# lenient_parse
[](https://hex.pm/packages/lenient_parse)
[](https://hexdocs.pm/lenient_parse/)
A Gleam library that replicates the functionality of Python's built-in `float()`
and `int()` functions for parsing strings into float and integer values. This
package offers more flexible parsing than the standard Gleam functions.
- `float("3.14")` -> `"3.14" |> lenient_parse.to_float`
- `int("42")` -> `"42" |> lenient_parse.to_int`
- `int("1010", base=2)` -> `"1010" |> lenient_parse.to_int_with_base(2)`
## Installation
```sh
gleam add lenient_parse
```
## Usage
```gleam
import gleam/float
import gleam/int
import gleam/io
import lenient_parse
pub fn main() {
// Parse a string containing an integer value into a float
"1" |> lenient_parse.to_float |> io.debug // Ok(1.0)
"1" |> float.parse |> io.debug // Error(Nil)
// Parse a string containing a negative float
"-5.001" |> lenient_parse.to_float |> io.debug // Ok(-5.001)
"-5.001" |> float.parse |> io.debug // Ok(-5.001)
// Parse a string containing a complex float with scientific notation
"-1_234.567_8e-2" |> lenient_parse.to_float |> io.debug // Ok(-12.345678)
"-1_234.567_8e-2" |> float.parse |> io.debug // Error(Nil)
// Parse a string containing an integer
"123" |> lenient_parse.to_int |> io.debug // Ok(123)
"123" |> int.parse |> io.debug // Ok(123)
// Parse a string containing a negative integer with surrounding whitespace
" -123 " |> lenient_parse.to_int |> io.debug // Ok(-123)
" -123 " |> int.parse |> io.debug // Error(Nil)
// Parse a string containing an integer with underscores
"1_000_000" |> lenient_parse.to_int |> io.debug // Ok(1000000)
"1_000_000" |> int.parse |> io.debug // Error(Nil)
// Parse a string containing a binary number with underscores
"1000_0000" |> lenient_parse.to_int_with_base(2) |> io.debug // Ok(128)
"1000_0000" |> int.base_parse(2) |> io.debug // Error(Nil)
// Parse a string containing a hexadecimal number with underscores
"DEAD_BEEF" |> lenient_parse.to_int_with_base(16) |> io.debug // Ok(3735928559)
"DEAD_BEEF" |> int.base_parse(16) |> io.debug // Error(Nil)
// Use base 0 to automatically detect the base when parsing strings with prefix indicators
let dates = [
"0b11011110000", "0o3625", "1865", "0x7bc", "0B11110110001", "1929",
"0O3507", "0X7a9", "0b11011111011",
]
dates
|> list.map(lenient_parse.to_int_with_base(_, 0))
|> io.debug()
// [Ok(1776), Ok(1941), Ok(1865), Ok(1980), Ok(1969), Ok(1929), Ok(1863), Ok(1961), Ok(1787)]
dates
|> list.map(int.base_parse(_, 0))
|> io.debug()
// [Error(Nil), Error(Nil), Error(Nil), Error(Nil), Error(Nil), Error(Nil), Error(Nil), Error(Nil), Error(Nil)]
// Nice errors
"12.3e_3" |> lenient_parse.to_float |> io.debug // Error(InvalidUnderscorePosition(5))
"12.3e_3" |> float.parse |> io.debug // Error(Nil)
}
```
## [Rigorous Testing](https://github.com/JosephTLyons/lenient_parse/tree/main/test/data)
`lenient_parse`'s testing is extensive. We test the tokenization step, the
overall parse procedure, as well as various intermediate layers. We currently
have ~500 passing tests. Regressions are **not** welcome here.
### Backed by Python
Each test input is also processed using Python's (3.13) `float()` and `int()`
functions. We verify that `lenient_parse` produces the same output as Python. If
Python's built-ins succeed, `lenient_parse` should also succeed with identical
results. If Python's built-ins fail to parse, `lenient_parse` should also fail.
This ensures that `lenient_parse` behaves consistently with Python's built-ins
for all supplied test data. Tests are run against both the Erlang and JavaScript
targets.
If you run into a case where `lenient_parse` and Python's built-ins disagree,
please open an [issue](https://github.com/JosephTLyons/lenient_parse/issues) -
we aim to be 100% consistent with Python's built-ins and we will fix any
reported discrepancies.
## Development
To run the tests for this package, you'll need to [install
`uv`](https://docs.astral.sh/uv/getting-started/installation/).
