[erlang-questions] Avoiding case nesting

Richard O'Keefe ok@REDACTED
Tue Sep 29 04:25:11 CEST 2009

Let's generalise slightly:

We are given a sequence of strings [S1,...,Sn].
 From this we want to generate a function F
such that F(S) = {i,R} if i is the smallest integer
for which S == Si ++ R, or F(S) = no_match otherwise.

If we knew these strings at program construction time,
we could just write

	f(S1 ++ R) -> {1,R};
	f(Sn ++ R) -> {n,R};
	f(_) -> no_match.

This suggests one way to proceed: just write this text
out to a file (with a suitable module header), compile
it, load it, and use it.  Thanks to hot loading, you can
repeat the process whenever you want to change the sequence
of strings.

At the other extreme,

seek(Target, [Prefix|Prefixes], I) ->
     case lists:prefix(Prefix, Target)
       of true -> {I, lists:nthtail(length(Prefix), Target)}
        ; false-> seek(Target, Prefixes, I+1)
seek(_, [], _) ->

searcher(Prefixes) ->
     fun (Target) -> seek(Target, Prefixes, 1) end.

The first trades high preprocessing time for low run time;
the second trades high run time for low preprocessing time.

Somewhere in the middle you could construct a ternary search
tree.  In a language without run-time code generation, a
ternary search tree would probably be the best way to go.

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