From Erlang Community

Revision as of 20:56, 17 August 2006

Testing Whether an Object is a String

Problem

You need to test if an object is a string. Since Erlang does not have objects per se, the goal here is really that you want to test if a term is a string.

Solution

You can't! An Erlang string is just a list of characters (really, a list of integer values). So, the variable you are examining could be a list of letters, a set of coordinates, or a set of bytecode for Donald Knuth's MMIX machine.

About the best you can do is determine that the term you are evaluating is a list or not. For that, use the Erlang built-in-function is_list:

1> A="This is fun!".
"This is fun!"
2> is_list(A).
true
3> B=12.
12
4> is_list(B).
false

If you were very concerned that a term was printable, you could verify that it consists of a list of integers that fall within the range of characters you consider to be a string:

1> Fun = fun(XX) ->         
1>     if XX < 0 -> false;  
1>        XX > 255 -> false;
1>        true -> true      
1>     end                  
1> end.
#Fun
2> Is_String = fun(XY) ->            
2>     case is_list(XY) of           
2>         false -> false;           
2>         true -> lists:all(Fun, XY)
2>     end                           
2> end.
#Fun
3> A.
"This is fun!"
4> Is_String(A).
true
5> C. 
[2000,1982,199891]
6> Is_String(C).
false