Comprehending List Comprehensions

[Hakan Mattsson]

On Mon, 26 Nov 2001, Peter Caven wrote:

Peter> I am at the early stages of learning Erlang and I've encountered what seems
Peter> to be strange behaviour in list comprehensions. The expression:
Peter> 
Peter> [{Token} || {Start, Length} <- [{1,1},{2,2},{4,2}], Token <-
Peter> string:substr("1+1-2", Start, Length)]
Peter> 
Peter> returns:
Peter> 
Peter> [{49},{43},{49},{45},{50}]
Peter> 
Peter> when I expected it to return:
Peter> 
Peter> [{"1"},{"+1"},{"-2"}]
Peter> 
Peter> Can someone please point out where my mistake is?

The Token generator with contribute with a solution for each element
in the list that string:substr/2 returns. See this little example:

    5> [Token  || Token <- string:substr("1+1-2", 1, 1)].
    "1"
    6> [{Token}  || Token <- string:substr("1+1-2", 1, 1)].
    [{49}]
    7> [{Token}  || Token <- string:substr("1+1-2", 1, 2)].
    [{49},{43}]
    8> [{Token}  || Token <- string:substr("1+1-2", 1, 3)].
    [{49},{43},{49}]

You problably intended to do this:

9> [{string:substr("1+1-2", Start, Length)} || {Start, Length} <- [{1,1},{2,2},{4,2}]].
[{"1"},{"+1"},{"-2"}]
10> 

/Håkan

---
Håkan Mattsson
Ericsson
Computer Science Laboratory
http://www.ericsson.com/cslab/~hakan/