Hoogle and the Rarity of Functions

There are only two hard things in Computer Science: cache invalidation and naming things. — Phil Karlton

Say what you want about Haskell, Hoogle is an amazing tool. If you don’t know, it’s simply a search engine for functions where your search term is a function signature. That probably sounds pretty plain. Also, my instinct was that since a function can do anything it likes with it’s inputs so long as the return type is correct, how useful can the signature alone really be? It turns out that for a given function signature, there are usually very few commonly used functions which match. What tends to happen is that the first one you see from Prelude (the Haskell standard library) is the one you want. As a toy example, imagine you are building a parsing library and the first step is to identify all the pronumerals: you want the “x” in “420x”. So you could do dropWhile isDigit "420x" ( Before you get upset, I know that this is not how to build a parser; it’s just a demonstration :melting face: ) I am far from an expert programmer, but I do know the take, drop, takeWhile etc functions well enough, probably because the names themselves are informative.

As the project develops, you might need to find the coefficients too, i.e. the “420” in this case. Since dropWhile skims over that part, you might wonder if there is a function that can do takeWhile and dropWhile in one go. Well if it exists, it is a function which takes a string and a predicate and returns two strings, which would most naturally be returned as a tuple. If we generalise slightly and think of lists of a generic type rather than lists of chars ( since that is all a string is anyway ) the the full signature must be [a] -> (a -> Bool) -> ([a], [a]). If you search for this signature in hoogle you get two functions from Prelude. One is called span and does exactly what we need. The other is called break which is almost the same, but finds the longest prefix where the predicate fails (rather than succeeds). I personally find the function names span and break entirely uninformative, but I don’t blame the authors of the Prelude for that: sometimes it is just really hard to find a good name for a function. That being the case, imagine how annoying it would be to try and find this function by any other means. Where would you even start? Looking at the documentation for the the list type? Ok, you would find it eventually for sure but it is really lovely to be able to get it instantly just by knowing the input and return types.

Finally the fact mentioned earlier about there only being a small number of functions for a given signature, which is what makes hoogle work in the first place, is an interesting one to me and I suppose is what this post is ultimately about.