Handlebars helpers within Rocket - rust

Question

I am trying to register a helper for handlebars the ultimate goal is to be able to have a helper that calculates/prints the length of a Vec. But I already fail to get the samples from the documentation into my program. I managed to do a 'minimal' example that shows the same error as my rocket page.
#[macro_use]
extern crate handlebars;
extern crate rocket;
extern crate rocket_contrib;
use handlebars::{Context, Handlebars, Helper, HelperResult, JsonRender, Output, RenderContext};
use rocket_contrib::templates::Template;
handlebars_helper!(hex: |v: i64| format!("0x{:x}", v));
fn wow_helper(
h: &Helper,
_: &Handlebars,
_: &Context,
_: &mut RenderContext,
out: &mut Output,
) -> HelperResult {
if let Some(param) = h.param(0) {
out.write("<b><i>")?;
out.write(&param.value().render())?;
out.write("</b></i>")?;
}
Ok(())
}
fn main() {
rocket::ignite()
.attach(Template::custom(|engines| {
engines
.handlebars
.register_helper("mlenb", Box::new(wow_helper));
}))
.launch();
println!("Hello, world!");
}
with the dependencies in cargo.toml:
[dependencies]
handlebars="3.3.0"
rocket="0.4.5"
[dependencies.rocket_contrib]
version = "0.4.5"
default-features = false
features = ["diesel_sqlite_pool", "handlebars_templates", "serve"]
The error:
error[E0631]: type mismatch in function arguments
--> src/main.rs:184:43
|
162 | / fn wow_helper(
163 | | h: &handlebars::Helper,
164 | | _: &handlebars::Handlebars,
165 | | _: &handlebars::Context,
... |
175 | | Ok(())
176 | | }
| |_- found signature of `for<'r, 's, 't0, 't1, 't2, 't3, 't4, 't5, 't6, 't7> fn(&'r handlebars::Helper<'s, 't0>, &'t1 handlebars::Handlebars<'t2>, &'t3 handlebars::Context, &'t4 mut handlebars::RenderContext<'t5, 't6>, &'t7 mut (dyn handlebars::Output + 't7)) -> _`
...
184 | .register_helper("mlenb", Box::new(wow_helper));
| ^^^^^^^^^^^^^^^^^^^^ expected signature of `for<'r, 'reg, 'rc, 's, 't0> fn(&'r rocket_contrib::templates::handlebars::Helper<'reg, 'rc>, &'reg rocket_contrib::templates::handlebars::Handlebars, &'rc rocket_contrib::templates::handlebars::Context, &'s mut rocket_contrib::templates::handlebars::RenderContext<'reg>, &'t0 mut (dyn rocket_contrib::templates::handlebars::Output + 't0)) -> _`
|
= note: required because of the requirements on the impl of `rocket_contrib::templates::handlebars::HelperDef` for `for<'r, 's, 't0, 't1, 't2, 't3, 't4, 't5, 't6, 't7> fn(&'r handlebars::Helper<'s, 't0>, &'t1 handlebars::Handlebars<'t2>, &'t3 handlebars::Context, &'t4 mut handlebars::RenderContext<'t5, 't6>, &'t7 mut (dyn handlebars::Output + 't7)) -> std::result::Result<(), handlebars::RenderError> {wow_helper}`
= note: required for the cast to the object type `dyn rocket_contrib::templates::handlebars::HelperDef`

Answer 1

Don't need to use crate handlebars in cargo.toml with rocket crate.
its works well like that:
in cargo.toml only
[dependencies]
rocket = "0.4.5"
[dependencies.rocket_contrib]
version = "0.4.5"
default-features = false
features = ["handlebars_templates", "serves"]
and in the main.rs use:
use rocket_contrib :: templates :: {Template, handlebars};
use handlebars :: {Helper, Handlebars, Context, RenderContext, Output, HelperResult, RenderError};
So there will be no problems with versioning the handlebars!

Answer 2

Thanks to #jebrosen:mozilla.org in the rocket matrix channel the problem got resolved.
Problem:
I used incompatible versions of handlebars. The rocket library uses version 1.0 while I used Version 3.0 in the Cargo.toml.
Solution:
Change the Cargo.toml to:
[dependencies]
handlebars="1.0"
rocket="0.4.5"
[dependencies.rocket_contrib]
version = "0.4.5"
default-features = false
features = ["diesel_sqlite_pool", "handlebars_templates", "serve"]
Careful a rebuild requires a cargo clean otherwise the changes don't get built.
Solution for the initial Problem:
I did not manage to use the macro handlebars_helper! to create the length helper but instead created it using the function syntax.
fn mlen_helper_fun(
h: &handlebars::Helper<'_, '_>,
_: &handlebars::Handlebars,
_: &handlebars::Context,
_: &mut handlebars::RenderContext<'_>,
out: &mut dyn handlebars::Output,
) -> handlebars::HelperResult {
if let Some(param) = h.param(0) {
if let serde_json::value::Value::Array(arr) = param.value() {
out.write(&format!("{}", arr.len()))?;
Ok(())
} else {
Err(handlebars::RenderError::new("Not a array"))
}
} else {
Err(handlebars::RenderError::new(
"No parameter given at least one required",
))
}
}
This can then be used with:
.attach(Template::custom(|engines| {
engines
.handlebars
.register_helper("length", Box::new(mlen_helper_fun));
}))