The function groupBy from haskell’s standard library is defined in terms of span, the effect being that the supplied predicate function is used to compare the first element of a group with successive elements.
This isn’t clear from the docs, and you might try to do this and wonder at the output you got:
*Main> groupBy (< =) [3,4,5,3,2,1,4,4,1,1,2,3,4,5,4,5,6,7] [[3,4,5,3],[2],[1,4,4,1,1,2,3,4,5,4,5,6,7]]
I’ve been thinking quite a bit lately about a category of functions that are always a bit awkward to define: they involve cases where we would like to traverse a recursive data structure and do something with the data that we have passed over but which is “gone now”.
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I just finished an initial release of an interpreter for the befunge programming language, based on the ‘93 spec. The project was quite fun! My goal was to produce a well-designed program with performance that didn’t suck too bad. Here are some highlights:
I found that writing the core functionality of the interpreter took almost no time, once I settled on the approach I would take. I used a monad transformer for the first time, StateT:
type REPL a = StateT ProgramState IO a
This let me pass around the state of the computation in the State monad while doing IO actions. This made some potentially-awkward befunge commands really easy to implement.
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