=> (last (range 0.0 10000000.0 0.1))
9999999.98112945
=> (last (range' 0.0 10000000.0 0.1))
9999999.9
Range is incremental by design, and that is how floats work. Something with the desired behavior would need to be a different fn with a different name.
"Returns a lazy seq of nums from start (inclusive) to end (exclusive), by step"
What specifically about that wording specifically suggests that the implementation will use naive increment-and-recurse behaviour? My reading is that the function will return a lazy sequence of numbers from start to end separated by step, not separated by 'almost step'.
This implementation leads to unexpected behaviour with bounds:
=> (count (range 0 100 1))
100
=> (count (range 0 10 0.1))
101
range could avoid this issue cleanly by converting floats to bigdecimals (let me know if you think this is a good idea?)
I ran into this problem recently, and have to say it was pretty ugly. This is how I avoided the issue:
(defn rangef
"Returns a sequence of n numbers from start to end inclusive."
[start end n]
(for [i (range 0 n)]
(+ start (* i (/ (- end start) (dec n))))))
Hope that helps any disillusioned float users out there, or just pass in BigDecimals to range instead of floats... I would go so far as to say using floats with range as it stands is almost always going to end in tears (or worse as Stephen describes 
).
[and just to be clear if it is considered an error, it would be nice if range explicitly forbade it]