Erlang, replacing an atom with another one in a list -

i want write function replace specific atom given atom in input list. want using pattern matching , not using conditional statements. idea?

and want write function return unique atoms in expression. e.g.

input:

[a, b, c, a, b]    

output:

c 

input:

[b, b, b, r, t, y, y]    

output:

[t, r] 

assuming want replace instances , keep order of list (works terms):

replace(old, new, list) -> replace(old, new, list, []).  replace(_old, _new, [],           acc) -> lists:reverse(acc); replace(old,  new,  [old|list],   acc) -> replace(old, new, list, [new|acc]); replace(old,  new,  [other|list], acc) -> replace(old, new, list, [other|acc]). 

for unique elements filter, need keep state of elements have looked @ already.

it awkward implement such function using pattern matching in function headers , not gain (performance) it. awkwardness come having loop through both list in question , list(s) keeping state of parsed elements. loose lot of readability.

i recommend going simpler (works terms, not atoms):

unique(list) -> unique(list, []).  unique([], counts) ->     lists:foldl(fun({e, 1}, acc) -> [e|acc];                    (_,      acc) -> acc                 end, [], counts); unique([e|list], counts) ->     unique(list, count(e, counts).  count(e, [])            -> [{e, 1}]; count(e, [{e, n}|rest]) -> [{e, n + 1}|rest]; count(e, [{x, n}|rest]) -> [{x, n}|count(e, rest)].