exception error: no match of right hand side value [] - erlang

Question

I am trying to see if a letter exists in a list, i am currently doing like the following
sortedlist(Text)-> freq(lists:sort(Text)).
freq(List) -> freq (List, [], []).
freq(List, Freq, CheckedLetters) when length(List) > 0 ->
[CurrHead|T]= List,
Checked = lists:member(CurrHead,CheckedLetters),
case Checked of
false -> CheckedLetters++[CurrHead],
freq(T,Freq++[{CurrHead,count(CurrHead, List)}],CheckedLetters);
true -> freq(T,Freq,CheckedLetters)
end;
freq([],Freq,CheckedLetters)-> Freq.
List contains user entered letters
CheckedLetters is an empty list, that will keep track of the already examined letters
But I am receiving the following erlang exception at the case-statement line (the lines with **).
error: no match of right hand side value []
What is the problem here? I am been blindy staring at those lines.

Answer 1

The end condition is missing in your recursive function. So when list become empty (each time you recall the function with the tail it is getting smaller), the match [CurrHead|T]= List, fails with error.
You have to add a clause to manage the end of recursion:
freq([], Freq, CheckedLetters) when length(List) > 0 ->
{Freq, CheckedLetters};
freq(List, Freq, CheckedLetters) when length(List) > 0 ->
[CurrHead|T]= List,
Checked = lists:member(CurrHead,CheckedLetters),
case Checked of
true -> CheckedLetters++[CurrHead],
freq(T,Freq++[{CurrHead,count(CurrHead, List)}],CheckedLetters);
false -> freq(T,Freq,CheckedLetters)
end;
I think you will have also to review the inside operation, I doubt it is doing what you expect. in particular the line CheckedLetters++[CurrHead], has no effect.
[edit]
Don't forget that variables are not mutable in erlang. So the line CheckedLetters++[CurrHead] simply evaluates a new list and forget the result (in fact I am not sure it does anything since you do not bound this evaluation to any variable, and the compiler knows it).
I guess that what you want to do is:
case Checked of
true ->
freq(T,Freq++[{CurrHead,count(CurrHead, List)}],CheckedLetters++[CurrHead]);
false ->
freq(T,Freq,CheckedLetters)
end;
Next you should have a look at Freq++[{CurrHead,count(CurrHead, List)}], I think it isn't correct.

Answer 2

The only piece of code in that block that could error: no match of right hand side value [] is [CurrHead|T]= List, as Steve Vinoski said. Likely there is some scenario where List is an empty list instead of a list of letters.
Also, the last line in the code block you shared:
case Checked == false of
...
end
Can be simplified to:
case Checked of
true ->
...
false ->
...
end
Since Checked is the return value of lists:member/2, and lists:member/2 is guaranteed to return a boolean, you can safely match directly on the true and false atoms.
lists:member/2 documentation: http://erlang.org/doc/man/lists.html#member-2