[erlang-questions] problem making my first parser

[zxq9]

On Saturday 26 September 2015 09:01:30 Roelof Wobben wrote:
> Hello,
> 
> I try to make a very easy parser to practice to make a json parser in 
> the future.
> As first I try to make a parser which can calculate some basic things.
> 
> So far I have this :
> 
> -module(number_parser).
> 
> -export([scan/1]).
> 
> scan(String) ->
>      scan(String, []).
> 
> scan([], List) ->
>      List;
> 
> scan([Head | Rest], List_parse) when Head >= $0, Head =< $9 ->
>       scan(Rest, [(Head - 48) | List_parse]);
> 
> scan([Head | Rest], List_parse) when Head =:= "+" ->
>     scan(Rest, [Head | List_parse] ).
> 
> 
> but when I try this code on scan("1+1") I see this error message :
> 
> ** exception error: no function clause matching 
> number_parser:scan("+1",[1]) (number_parser.erl, line 8)
> 
> Which does not make sense. +1 makes as far as I know [+, 1] so I have a 
> head and tail so the last clause schould work.
> 
> 
> Why do I then see this error message ?

What happens if you try `[1] >= $0, [1] =< $9.`?

How about `[1] =:= "+".` ?

What are you actually comparing here?

Do you care about plus signs as strings, or do you care about plus as a symbol that represents an operation?

-Craig