[erlang-questions] Typespecs of functions that may fail

[Bastien Chamagne]

Makes perfect sense! Thank you Kostis for this explanation.

I'll dig deeper.



On 01/17/2017 04:52 PM, Kostis Sagonas wrote:
> On 01/17/2017 03:47 PM, Bastien Chamagne wrote:
>> Hello,
>>
>> I'm trying to use dialyzer and I don't know how to type functions that
>> are written in "happy path" (may crash).
>
> Dialyzer's core (known as "success typings") are designed for 
> _exactly_ this purpose.  So that you only specify what's happening in 
> the "happy paths", i.e., in the paths that a function returns 
> ("succeeds").  So you cannot (and should not) specify anything 
> regarding the "sad paths."
>
>
>> Here's a sample function that may or may not crash depending if Field
>> correspond to a relation field on given Model:
>>
>>     -spec is_relation(field(), model()) -> {true, relation_type()} | 
>> false.
>>
>>     %% @doc Sets a relation on the given model.
>>     -spec set_relation(field(), model(), model()) -> model() | 
>> no_return().
>>     set_relation(Field, RelatedModel, Model) ->
>>         % On next line, I'm OK that this function crashes if I try to
>> set a relation on a non-relation field
>>         {true, RelationType} = is_relation(Field, Model),
>>         set_relation(Field, RelatedModel, Model, RelationType).
>>
>> Here's what dialyzer says:
>>
>>     model.erl:180: Function set_relation/3 has no local return
>>     model.erl:181: The pattern {'true', RelationType} can never match
>> the type 'false'
>>
>> Is there anything that I can do to inform dialyzer that this is a OK
>> behaviour for me?
>
> This behaviour should NOT be OK for you.  Dialyzer is telling you that 
> it has somehow discovered that it your is_relation/2 function call 
> always returns 'false' here.  Unless your version of Dialyzer has some 
> bug, which is unlikely, you cannot ignore that this warning.
>
> In any case, the problem is not in the set_relation/2 function but in 
> the is_relation/2 function for which you have not showed us its code; 
> actually, you need to show us the whole module in order to tell you 
> with certaintly what's happening.
>
>
> Kostis
>
> PS. Types of the form  type() | no_return()  are equally verbose as 
> writing 42 + 0 for the integer 42 ...
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