CODEWITHSUNDEEP
erlang
ankush981
ankush981

Reputation: 5417

Our own tuple_to_list() function

I'm required to write my own tuple_to_list() function (yes, from the book) and came up with this in my erl file:

%% Our very own tuple_to_list function! %%
% First, the accumulator function
my_tuple_to_list_acc(T, L) -> [element(1, T) | L];
my_tuple_to_list_acc({}, L) -> L;
% Finally, the public face of the function
my_tuple_to_list(T) -> my_tuple_to_list_acc(T, []).

When I compile this, however, I get the following error in the shell:

28> c(lib_misc).
lib_misc.erl:34: head mismatch
lib_misc.erl:2: function my_tuple_to_list/1 undefined
error

I have no clue what "head mismatch" there is, and why is the function undefined (I've added it to the module export statement, though I doubt this has much to do with export statements)?

Upvotes: 1

Views: 70

Answers (3)

Alexey Romanov
Alexey Romanov

Reputation: 170899

The other answer explains how to fix this, but not the reason. So: ; after a function definition clause means the next clause continues the definition, just like as for case and if branches. head mismatch means you have function clauses with different names and/or number of arguments in one definition. For the same reason, it is an error to have a clause ending with . followed by another clause with the same name and argument count.

Changing the order of the clauses is needed for a different reason, not because of the error. Clauses are always checked in order (again, same as for case and if) and your first clause already matches any two arguments. So the second would never be used.

Upvotes: 2

Hynek -Pichi- Vychodil
Hynek -Pichi- Vychodil

Reputation: 26131

When you are interested in working tuple_to_list/1 implementation

1> T2L = fun (T) -> (fun F(_, 0, Acc) -> Acc; F(T, N, Acc) -> F(T, N-1, [element(N, T)|Acc]) end)(T, tuple_size(T), []) end.
#Fun<erl_eval.6.50752066>
2> T2L({}).
[]
3> T2L({a,b,c}).
[a,b,c]

Or in module

my_typle_to_list(_, 0, Acc) -> Acc;
my_typle_to_list(T, N, Acc) ->
    my_typle_to_list(T, N-1, [element(N, T)|Acc]).

my_typle_to_list(T) ->
    my_typle_to_list(T, tuple_size(T), []).

Note how I use decreasing index for tail recursive function.

Upvotes: 1

P_A
P_A

Reputation: 1818

Those errors mean that you didn't end definition of my_tuple_to_list_acc/2. You should change order of first two code lines and add dot after them.

my_tuple_to_list_acc({}, L) -> L;
my_tuple_to_list_acc(T, L) -> [element(1, T) | L].

Upvotes: 2

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