Support for sub-expressions

[breenyoung]

I have a {{#each}} loop and within each iteration I have a helper being called I'd like to pass the @index property to. Is it possible? Template compilation fails when I try to pass it into a helper (without the curly braces:

{{#each }}
{{myHelper @index arg2}}
{{/each}}

[rexm]

This should be possible. Looks like a bug

[rexm]

Hm, I ran the below test case without any problems. Can you compare? One thing I noticed is that your code snippet doesn't include the name of the object you're each-ing.

string source = "{{#each people}}{{myHelper @index name}}{{/each}}";

Handlebars.RegisterHelper("myHelper", (output, context, arguments) =>
    {
        output.Write("{0}. {1} ", arguments[0], arguments[1]);
    });

var template = Handlebars.Compile(source);

var data = new {
    people = new [] {
        new {
            name = "Bill"
        },
        new {
            name = "Mary"
        }
    }
};

var result = template(data);
Assert.AreEqual("0. Bill 1. Mary ", result);

[breenyoung]

Sorry I omitted some of the code to be brief. It was actually in a sub expression I was trying to use it in.

{{#each InsuranceItems}}
{{#ifCond (mathHelper @index "%" "2") "ne" "0"}}

{{InsuranceValue}} {{/ifCond}} {{/each}}

InsuranceItems is an IList of custom objects and mathHelper is just a custom helper that takes 3 arguments to execute a math function (in this case mod). IfCond is just an enhanced IF that can take a condition and test value.

[rexm]

The parens are causing the issue. The parser does not understand those characters, and the compiler doesn't support sub-expressions generally.

Sent from my iPhone

On Apr 20, 2015, at 8:07 AM, Breen notifications@github.com wrote:

Sorry I omitted some of the code to be brief. It was actually in a sub expression I was trying to use it in.

{{#each InsuranceItems}}
{{#ifCond (mathHelper @index "%" "2") "ne" "0"}}

{{InsuranceValue}}

{{/ifCond}}

{{/each}}

InsuranceItems is an IList of custom objects and mathHelper is just a custom helper that takes 3 arguments to execute a math function (in this case mod). IfCond is just an enhanced IF that can take a condition and test value.

—
Reply to this email directly or view it on GitHub.

[breenyoung]

Ah ok, is sub expressions something on the table to implement in the future?

[rexm]

My desire is to support those in v2, but there's no timeframe for that yet. If someone took on that feature it'd get us a lot closer sooner.

[rexm]

Support for sub-expressions is desired for v2. See the Subexpressions section on the handlebarsjs documentation: http://handlebarsjs.com/expressions.html

[rexm]

This will require 2 main changes:

• Enhancing the lexer & converter to understand parentheticals and convert them to a new expression type that behaves similarly to PathExpression... this part will be fairly straightforward
• Allowing helpers to optionally return a value. Only having given this 5 minutes of thought, it doesn't seem so straightforward. Two possible solutions, neither of which I like:
  1. Break the HandlebarsHelper delegate contracts to return object instead of void. This is a huge breaking change and imposes an extra burden on writing every helper, even though most won't need it.
  2. Pass a Func<object> into the helpers as Return so they can optionally be called as Return(myValue);. This is also a breaking change, imposes less of a burden, but is pretty far off in left field in terms of normal program flow.

Open to suggestions in how to handle the return value!

[hkouns]

I wanted to add that I have had a very similar experience that I just referenced in a pull request for another project. (https://github.com/bvcms/bvcms/pull/81)

I have written several helpers including:
IfCond in the form of {{#IfCond lval oper rval}} {{/IfCond}}

Compare in the form of {{Compare lval oper rval}}

Math in form of {{Math lval oper rval "fmt?"}}
ex. {{Math Total "/" 12 "C"}}
fmt is optional. Uses standard/custom dotnet format strings

So when I added the following line to the template, I expected the Math sub-expression to be calculated first and the result returned (from writer.Write(...) statement) and used as an input in the Compare Helper.

{{Compare (Math Total "/" 12) ">" 100}}

returns an error...while

{{Compare 200 ">" 100}}

returns "True"

Similarly If I used the Compare helper for a logical operation (AND)

{{Compare "True" "&&" "True"}}

returns "True" ... while

{{Compare (Compare 200 ">" 100) "&&" "True"}}

returns an error.

Unfortunately, I don't have a solution... so just adding my name to those interested.

[rexm]

Kind of an interesting approach that doesn't break the existing contracts to try to do a type coercion on the value written to the stream out of a helper.