the Patch-Tag blog

It is said that in haskell, If it Compiles, It Works.

This is true, but what do you do when it won’t compile?

I came across a real world scenario for this just now, when I was in the process of removing the HStringTemplateHelpers dependency from happs tutorial. The reason for this is that HStringTemplateHelpers uses the unix package (indirectly, via FileManip), which means that at the present time happs tut won’t run on windows, which lately has me smacking myself like dobby the masochistic house elf in the harry potter movies.*

The dependency wasn’t in one file, but scattered throughout the code. Some places, the dependency wasn’t that important, I could just comment the function out, but in other cases the function was core to the app and commenting it out caused cascading compile failures. Doing the type/function arithmetic in my head for figuring out what depended on what was giving me a headache and tempting me to veg out on youtube rather than face the problem, always a warning sign for me that I’m doing something wrong.

I needed to make my program compile, just so I could think about it with my head screwed on straight.

So, instead of commenting out the HStringTemplateHelpers-dependent functions, I set them equal to undefined and commented out their type signatures (since the type sig might be using a type that was defined in the dependency.


-- paintProfile :: RenderGlobals -> String -> UserProfile -> String -> String
paintProfile rglobs user cp userimagepath = undefined {-.....-}


A few comment-outs later and cabal install compiles! Of course, if I actually try running the resulting binary, I will hit an undefined error right away, but the point is that I can think again, and start rewriting my offending functions in a methodical way until there are no more undefineds, which is easly checked with

grep -irn undefined src

For what it’s worth, I could have also used

paintProfile rglobs user cp userimagepath = error "paintProfile uses HStringTemplateHelpers... bad dobby! bad dobby! Dobby will have to shut his ears in the oven door for this."

But whatever

* I suppose the righter thing to do would be to fix FileManip, but I am choosing the easy way out just to get it done. Eventually I woud like to move away from HStringTemplate with happs and use mostly Text.XHTML and/or HSP, which is type safe, which HSTringTemplate just thrown in for convenience and newb friendliness.