[erlang-questions] Tried this... Thought it interesting... But it didn't work.

Lloyd R. Prentice <>
Wed Sep 2 04:24:46 CEST 2015


Hi Jesper,

Your code exhibits a depth of understanding and subtlety that I can only hope to aspire to.

Can you help me understand the ?LET(... and ?SUCHTHAT(... expressions? 

As always, I deeply appreciate your guidance.

Lloyd

Sent from my iPad

> On Sep 1, 2015, at 2:01 PM, Jesper Louis Andersen <> wrote:
> 
> 
>> On Tue, Sep 1, 2015 at 6:43 PM, zxq9 <> wrote:
>> Honestly, I don't really think this is the best use of list comprehension syntax. I can easily imagine people being confused at that, or at least requiring a few moments thought to figure out wtf is going on in that line. The fact this confounded you (and the example was unclear in the blog post, imo) is a good reason why you shouldn't do this in your code.
> 
> I must admit I sometimes do this. Consider:
> 
> f(X) ->
>     E = case X > 5 of
>         true -> [7];
>         false -> []
>     end,
>     [1,2,3] ++ E ++ [4,5].
> 
> This is easier written as
> 
> f(X) ->
>    [1,2,3] ++ [7 || X > 5] ++ [4,5].
> 
> but as a way to get the list comprehension to append like this is the only way I tend to use the construction. For real-world use, consider the following snippet from my Maps R18 tests:
> 
> https://github.com/jlouis/maps_eqc/blob/96d06da56053e87dd33c830b293dface525be17d/src/maps_eqc.erl#L693-L696
> 
> remove_args(#state { contents = C } = State) ->
>     frequency(
>       [{5, ?LET(Pair, elements(C), [element(1, Pair)])} || C /= [] ] ++
>       [{1,  ?SUCHTHAT([K], [map_key(State)], find(K, 1, C) == false)}]).
> 
> The idea here is that we want to generate arguments for removing an element from a map, and C contains the current contents of the map. If the map is non-empty, C /= [], and we can pick elements from C. Otherwise, we generate a map key such that it is really not an element of the map in question (which is trivially true if C = []). The neat part is that the first variant with frequency 5 is never generated for the empty map.
> 
> A case analysis in this case would tend to repeat code, so I find this somewhat more nimble. But these situations are probably the only situations on which I use this way of writing.
> 
> 
> -- 
> J.
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> 
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