[Haskell-cafe] Re: Strange space leak

[Jacques Carette]

apfelmus wrote:
> Grzegorz Chrupala wrote:
>>>
>>>    split "<DOC>" . words . map toLower = (:[]) . words . map toLower
>>>
>>> Since you converted everything to lowercase, the string "<DOC>" will 
>>> never appear in the text, resulting in a single huge document.
>>
>> Oops, that should have been obvious, sorry for the dumb question.
>> Thanks,
>
> No problem, it was not obvious to me and I had fun trying to figure it 
> out :)

Speaking of not obvious: Haskell's type system catches a lot of bugs -- 
but still gives no help with this particular 'problem'.  But one can 
easily imagine an extension to a type system which could have detected 
that "<DOC>" can never occur in the result of words . map toLower, and 
then with a bit more work [type-level Nat], the type of the full 
expression could have encoded that the result is always going to be of 
length 1.  That would surely have been a good hint that something 
non-trivial was going on.

Whether a Haskell-friendly type system extension could be 
created/implemented which would cover this example, I don't know.  
However, I have had a lot of fun with the underlying idea: anytime 
someone encounters a bug in their code (and relates the debugging story 
on haskell-cafe), try to imagine how the type system could be extended 
to automate that.  In most cases, I don't mean to have the type system 
reject the code, but rather to have an inferred type that would make it 
obvious that the code did not behave as expected.

Jacques