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 [email protected] 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.

[rexm]

@hkouns @breenyoung basic sub-expressions are now supported in 1.5.0. I appreciate any help you can provide to give it a try in your projects and feedback here.

Usage is as @hkouns described:

the sub-expression [evaluated] first and the result returned from writer.Write(...) statement used as an input in the [outer] helper.

In other words, if a helper is put inside a sub-expression (parentheticals), the output written to the buffer will be captured independently and passed as an argument into the outer helper expression.

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

The inner Compare helper will be evaluated with the arguments 200, ">", and 100; the outer Compare helper will then be evaluated with the arguments {TextWriter output from inner Compare}, "&&", and "True"

[gretro]

Test for SubExpressions are failing under Visual Studio 2015. Could it be an issue with .NET 4.6?

This is the StackTrace:

HandlebarsDotNet.HandlebarsCompilerException : An unhandled exception occurred while trying to compile the template
  ----> HandlebarsDotNet.HandlebarsCompilerException : An unhandled exception occurred while trying to compile the template
  ----> System.ArgumentException : Cannot bind to the target method because its signature or security transparency is not compatible with that of the delegate type.
   at HandlebarsDotNet.Compiler.FunctionBuilder.Compile(IEnumerable`1 expressions, String templatePath) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\FunctionBuilder.cs:line 66
   at HandlebarsDotNet.Compiler.HandlebarsCompiler.Compile(TextReader source) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\HandlebarsCompiler.cs:line 32
   at HandlebarsDotNet.Handlebars.HandlebarsEnvironment.Compile(TextReader template) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\HandlebarsEnvironment.cs:line 51
   at HandlebarsDotNet.Handlebars.HandlebarsEnvironment.Compile(String template) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\HandlebarsEnvironment.cs:line 58
   at HandlebarsDotNet.Handlebars.Compile(String template) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Handlebars.cs:line 29
   at HandlebarsDotNet.Test.SubExpressionTests.BasicSubExpression() in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars.Test\SubExpressionTests.cs:line 25
--HandlebarsCompilerException
   at HandlebarsDotNet.Compiler.FunctionBuilder.Compile(IEnumerable`1 expressions, Expression parentContext, String templatePath) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\FunctionBuilder.cs:line 53
   at HandlebarsDotNet.Compiler.FunctionBuilder.Compile(IEnumerable`1 expressions, String templatePath) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\FunctionBuilder.cs:line 61
--ArgumentException
   at System.Delegate.CreateDelegate(Type type, MethodInfo method, Boolean throwOnBindFailure)
   at System.Delegate.CreateDelegate(Type type, MethodInfo method)
   at HandlebarsDotNet.Compiler.SubExpressionVisitor.VisitSubExpression(SubExpressionExpression subex) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\Translation\Expression\SubExpressionVisitor.cs:line 27
   at HandlebarsDotNet.Compiler.HandlebarsExpressionVisitor.Visit(Expression exp) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\Translation\Expression\HandlebarsExpressionVisitor.cs:line 49
   at System.Linq.Expressions.ExpressionVisitor.Visit(ReadOnlyCollection`1 nodes)
   at System.Linq.Expressions.ExpressionVisitor.VisitNewArray(NewArrayExpression node)
   at HandlebarsDotNet.Compiler.HandlebarsExpressionVisitor.Visit(Expression exp) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\Translation\Expression\HandlebarsExpressionVisitor.cs:line 51
   at System.Linq.Expressions.ExpressionVisitor.VisitArguments(IArgumentProvider nodes)
   at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at HandlebarsDotNet.Compiler.HandlebarsExpressionVisitor.Visit(Expression exp) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\Translation\Expression\HandlebarsExpressionVisitor.cs:line 51
   at System.Linq.Expressions.ExpressionVisitor.VisitBlock(BlockExpression node)
   at HandlebarsDotNet.Compiler.HandlebarsExpressionVisitor.Visit(Expression exp) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\Translation\Expression\HandlebarsExpressionVisitor.cs:line 51
   at HandlebarsDotNet.Compiler.SubExpressionVisitor.Visit(Expression expr, CompilationContext context) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\Translation\Expression\SubExpressionVisitor.cs:line 12
   at HandlebarsDotNet.Compiler.FunctionBuilder.Compile(IEnumerable`1 expressions, Expression parentContext, String templatePath) in C:\Users\Gabriel\Desktop\rexm-Handlebars.Net\source\Handlebars\Compiler\FunctionBuilder.cs:line 47

[rexm]

Seems to be a difference between mono and Microsoft... fixed and pushed to Nuget 1.5.1

[rexm]

Haven't gotten any feedback... going to assume this is good. If you run into any problems, please open a new issue.

[aweintraut]

I realize this is a closed issue, apologies if I should open a new one (will do if requested). I have a helper "ResourceLookup" used to look up a resource string. But I need to build the name of the resource string from my object tree.

What syntax would I use? In this example I need to get myObj.Type formatted as an argument to the TypeName_ resource string, but I have to have the quotes around TypeName or it doesn't pass it as a string to my helper.

 {{ResourceLookup "TypeName_(myObj.Type)"}}

What I get right now is a string that directly contains "TypeName_(myObj.Type)"
If I leave off the quotes, I get an error - Message: Token ')' could not be converted to an expression.
Appending one more option - if I do {{ResourceLookup TypeName_{{myObj.Type}} }}, I get "undefined" as the value of arg1 in my helper.

Any help is appreciated, thanks.

[rexm]

Please open a new issue so we can keep the discussion focused. When you do that, can you please include the model you are using in this example?

[davidcarroll]

Thank you @rexm for adding the Sub-Expression functionality. I just started using it and it works great!

[esskar]

@rexm

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:
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.

Well, you could have two HandlebarsHelper delegates:
the first returns void (as it is);
the second returns an object (HandlebarsHelperWithResult); then, when HandlebarsHelper are registered, you can convert them to an HandlebarsHelperWithResult like
private static readonly NoValue = new object();

public void RegisterHelper(string helperName, HandlebarsHelper helperFunction) 
{
    RegisterHelper(helperName, (TextWriter output, dynamic context, params object[] arguments) => {
        helperFunction(output, context, arguments);
        return NoValue;
    });
}

This would not introduce any breaking changes.

[rexm]

If you continue the rest of the thread, you'll see we came to a solution and implemented it in 1.5