Clojure: sequences, mapping and the "current number/index" issue

Question

I process all files in a directory, say:

(map file-handler files)

The only parameter to file-handler is naturally a file object. I want to know inside the file-handler the current "index" of the files sequence. The only solution I can see so far is to create a counter in the lexical closure, visible inside file-handler, and increment it there on each step. Perfectly feasible but not quite clojurelike. Is there a pure functional way to manage it?

Very often in FP it turns out that you don't need an index even though you thought you did. If you do then as @noisesmith's said, the general principle is to pass the 'mutating' value through the recursion as one of the arguments, e.g. fold and reduce — shmish111
– shmish111, Commented Nov 20, 2014 at 22:05

noisesmith · Accepted Answer · 2014-11-19 06:59:51Z

3

map takes 1 or more arguments

(map file-handler (range) files)

there is also a shortcut for this

(map-indexed file-handler files)

This will require updating file-handler to take 2 arguments, or making a wrapper function that takes the index and the file and calls file-handler on the file.

answered Nov 19, 2014 at 6:59

noisesmith

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2 Comments

Alexey Orlov Over a year ago

Thank you very much, noisesmith! Neglected the RTFM part, shame on me.

noisesmith Over a year ago

as a general principle, in functional programming you can take any stateful variable, and turn it into an immutable function argument that gets passed forward.

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