April 22, 2010

GravatarBy Michael Snoyman

I had a pedantic moment I wanted to share with you all while working on web-routes-quasi. It should also serve as a nice introduction to this (not yet released) package.

This package consists of essentially two components: a quasi-quoter which converts a simple syntax into a [Resource], and some template haskell code that generate a URL datatype, render, parse and dispatch functions based on this [Resource]. For now, we'll focus on just the parse function. It takes a [String] and returns either a URL (of the aforementioned datatype) or an error message; the error message is always "Invalid URL", so nothing truly interesting there.

.... and yes, pun intended on nothing...

Anyway, the template haskell code to generate the parse function basically runs through the list of Resources, generates a clause for each one, and then tacks on a catchall clause at the end to return the error message.

What went wrong

Well, there's one little corner case that I hadn't thought of. Let's say that we are writing an application where every route is valid. Then that catch-all clause will trigger an overlapping pattern warning. This seems rather benign for two reasons:

  1. What web application says that every possible input is a valid route?
  2. You can always just disable the warning, or at least ignore it.

However, I came up against a case that defeated both reasons:

  1. I'm writing a subsite for Yesod which handles static file requests. In such a situation, every request is theoretically valid and must be checked against the filesystem.
  2. I'm pedantic, and hate all warnings.

The Goal

So the goal is simple: determine when a list of resources covers every possible request, and if so, simply omit the catch-all clause.

This is where you get to see some web-routes-quasi internals. A Resource is defined as:

data Resource = Resource String [Piece] Handler
data Piece = StaticPiece String
           | StringPiece String
           | IntPiece String
           | SlurpPiece String
data Handler = ByMethod [(String, String)]
             | Single String
             | SubSite String String String

The exact meanings of all of these constructors is well documented in the source code, so I will forgo it here. But basically, here's what's important for our purposes:

  • A slurp piece (which must be the last piece for a resource) collects all the remaining pieces in a request. A subsite does basically the same thing, passing all the remaining pieces to the subsite parser.
  • A string piece matches any single path segment.
  • Int piece matches only integral pieces; static piece matches a specific word. For purposes of determining completeness, any resource which includes a static or int piece must be thrown out. (Proof left as exercise to reader.)

So how do we determine whether a resource list is complete?

My Algorithm

  • Throw out resources with an int or static piece.
  • Count up the number of string pieces in each resource.
  • If the last piece is a slurp, or the handler is a subsite, consider this resource a "slurp resource".
  • Find the minimum number of string pieces for a slurp resource. If there exist non-slurp resources with string piece counts number from 0 to the minimum count for slurp resources (exclusive), then the resources are complete. Otherwise, they are incomplete.


  1. Why does my algorithm work?
  2. I implemented it in 22 lines of code. It could easily be shorter; have fun! It's the areResourcesComplete function in Web.Routes.Quasi. Just make sure the test suite passes. (Unless someone finds a bug or makes the program clearer, I won't be including patches; this code need not be performant since it's run compile-time.)
  3. Is there a better algorithm?


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