Question

I’m reading The Rails 3 Way and I don’t understand why there’s a proc in the routes. I still have a hard time grasping the use of proc/lambda and how they are used.

How is this:

match 'records/:id' => "records#protected",
  :constraints => proc {|req| req.params[:id].to_i < 100 }

different from:

match 'records/:id' => "records#protected",
  :constraints => params[:id].to_i < 100

?

Answer 1

Maybe you’re more familiar with JavaScript? Procs are similar to anonymous functions. The first version of your code is roughly equivalent to:

{ constraints: function(req) { return req.params["id"] < 100 }}

That is, constraints is being set to a block of code which can be evaluated at a later time.

The second version would be like writing

{ constraints: return req.params["id"] < 100 }

which is to say, a syntax error, and logically broken as it tries to evaluate return req.params["id"] < 100 immediately as the code is interpreted, before req has even been defined.

This is the primary difference between your two versions, and why the proc is important: You need to pass the code into the route so it can be evaluated later when routing occurs.