[erlang-questions] the semantics of dict:append/3

Robert Virding <>
Mon Sep 18 02:54:32 CEST 2006


I would say that seeing there is no value then it cannot be not a list 
of values. Ergo it shouldn't generate an error! :-) Orddict behaves the 
same way.

I was probably just being nice.

Robert

Ulf Wiger (TN/EAB) wrote:
> During a recent QuickCheck course, one of our guys (Johan Tjäder) started playing with applying QuickCheck to the dict.erl module. The following behaviour came as a surprise:
> 
> 1> D0 = dict:new().
> {dict,0,
>       16,
>       16,
>       8,
>       80,
>       48,
>       {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},
>       {{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]}}}
> 2> D1 = dict:append(a, 1, D0).
> {dict,1,
>       16,
>       16,
>       8,
>       80,
>       48,
>       {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},
>       {{[],[[a,1]],[],[],[],[],[],[],[],[],[],[],[],[],[],[]}}}
> 3> dict:find(a,D1).
> {ok,[1]}
> 
> 
> The documentation has this to say about dict:append/3:
> 
>   "This function appends a new Value to the current list
>    of values associated with Key. An exception is generated
>    if the initial value associated with Key is not a list
>    of values."
> 
> (http://www.erlang.org/doc/doc-5.5.1/lib/stdlib-1.14.1/doc/html/dict.html#append/3)
> 
> I would have thought that the initial value associated
> with a key not present in the dictionary is undefined -
> not the empty list. Shouldn't then the above experiment
> result in a badarg?
> 
> BR,
> Ulf Wiger
> 
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