Saturday, March 21, 2009

Imperative language constructs in Erlang

"How can I do for loop in Erlang?", "How can I do while loop in Erlang?"....

These are the common questions people ask when they get into the world of functional programming from imperative language(s). It is easy to switch to functional programming in Python, Ruby etc as they are the mixture of functional and imperative language constructs. Programming languages like Erlang (probably Haskell) do not provide explicit for, while constructs. Here are the equivalents way to implement some of the imperative language constructs.

For Loop

for_loop(N, Fun) ->
lists:foreach(Fun, lists:seq(1, N))

for_loop(0, Fun) -> ok;
for_loop(N, Fun) ->
for_loop(N-1, Fun).

While loop

while_loop(Predicate, Fun, State) ->
while_loop(Predicate(State), Predicate, Fun, State).

while_loop(PredResult, Predicate, Fun, State) when PredResult == false -> ok;
while_loop(_PredResult, Predicate, Fun, State) ->
NewState = Fun(State),
whle_loop(PredResult(State), Predicate, Fun, NewState).

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