# TryFrom Constructors

Often times we want to assure that various properties must be true about the arguments passed to a contract instance.

By using TryFrom and being careful with the visibility of inner fields it is possible to guarantee that the only way to get an X is by going through type Y.

This can be bound using the serde(try_from) attribute, which makes it so that any deserialization of X first passes through Y. This is particularly useful when X contains types (such as function pointers or caches) that cannot be deserialized, but we want to provide a way for a third party to pass JSON args to construct an X.


#![allow(unused)]
fn main() {
use std::convert::TryFrom;
use std::convert::TryInto;
use serde::*;
/// inner argument not pub, X cannot be constructed without going through Y
#[derive(Serialize, Deserialize, JsonSchema)]
#[serde(try_from="Y")]
pub struct X(u32);

#[derive(Serialize, Deserialize, JsonSchema)]
pub struct Y(pub u32);
impl TryFrom<Y> for X {
type Error = &'static str;
fn try_from(y: Y) -> Result<Self, Self::Error> {
if y.0 < 10 {
Err("Too Small I Guess?")
} else {
Ok(X(y.0))
}
}
}

let x: X = Y(10).try_into().unwrap();

}