CODEWITHSUNDEEP
erlanganonymous
Mahantesh
Mahantesh

Reputation: 79

Not able to assign fun to variable in a erlang module

I am playing around with example code from Programming with Erlang. I am struck with funs. help me understand whats going wrong with below code.

-export([totalcost/1]).

Costeach = fun(X) -> {W,Q} = X, shop:cost(W)*Q end.
sum(H|T) -> H + sum[T];
sum([]) -> 0.
totalcost(L) -> sum(lists:map(Costeach,L)).

All i wanted to achieve was being able to pass Variable (with fun assigned to it) as parameter to map. get total cost from this module

i keep getting syntax error/ Costeach not initialized error.

Upvotes: 4

Views: 271

Answers (1)

Dogbert
Dogbert

Reputation: 222408

You cannot define a function like that at the top level. You have two options: use the syntax you're using but declare it inside a named function, or declare it like a named function and use fun name/arity syntax to convert it to an anonymous function to pass into lists:map.

First approach:

sum(H|T) -> H + sum[T];
sum([]) -> 0.
totalcost(L) ->
  Costeach = fun(X) -> {W,Q} = X, shop:cost(W)*Q end.
  sum(lists:map(Costeach,L)).

Second approach:

costeach(X) -> {W,Q} = X, shop:cost(W)*Q.
sum(H|T) -> H + sum[T];
sum([]) -> 0.
totalcost(L) -> sum(lists:map(fun costeach/1,L)).

Tip: you can do the destructure in the function arguments in both cases to save a line of code:

Costeach = fun({W,Q}) -> shop:cost(W)*Q end.

or

costeach({W, Q}) -> shop:cost(W)*Q.

Upvotes: 6

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