[erlang-questions] "Design By Contract" in Erlang

Jesper Louis Andersen jesper.louis.andersen@REDACTED
Sun Jan 22 13:33:19 CET 2012

On 1/11/12 10:19 PM, Zabrane Mickael wrote:
> Hi guys,
> Any Erlang library to simply *DbC* (à la Eiffel)?
I tend to just do:

foo(Args) when GuardArgs ->
   true = assert_precondition(Args),
   Result = ...
   true = assert_postcondition(Result),

Where GuardArgs protects the inbound arguments as much as possible. The 
basic idea is that if your assumptions are not satisfied, you crash 
before you begin doing bad stuff. As long as your functions are pure, 
this works well. Also note that in many cases, providing such 
pre/post-conditions actually strengthens the dialyzer quite a bit since 
it uses the information to narrow down the valid control-flow/data-flow 
paths in the code.

That said, I am not consistent in writing code like this. I tend to 
assert certain situations now and then, especially if the code has 
proven to be tricky. In fact, if you look at typical erlang code,

{ok, R} = ...

is such an assertion. It protects the caller by asserting that a certain 
operation succeeded. I'll recommend code be built around this and then 
handling the crashes properly.

Jesper Louis Andersen
   Erlang Solutions Ltd., Copenhagen, DK

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