Added bnf when adding a new concept + Started logging filtering

docs/blog.rst

@@ -493,4 +493,108 @@ Even now that I am writing it, I just can't believe it. I must I have implemente
 it wrong. But the profiling shows that the time is lost in the under layers of the
 FS library.
 
-It's a shame !
+It's a shame !
+
+2019-12-01
+**********
+
+Using BNF to define concept
+"""""""""""""""""""""""""""""
+
+I always knew that there will be several ways to define the body of a concept (same
+goes for the 'pre', 'post' and 'where' parts). It can be defined as Python code,
+or something that is related to concepts. It can even be a new language that I will
+design. The important point, is that contrarily to traditional development languages,
+Sheerka must remain extensible.
+
+Same goes for the definition of the name.
+
+The traditional form is:
+
+::
+
+    def concept boo bar baz as ...
+
+So the concept is defined by the sequence 'foo', then 'bar' then 'baz'. In this order.
+
+Another way is
+
+::
+    def concept a plus b where a,b as ...
+
+In this form, a and b are supposed to be variables.
+It will be matched against :code:`one plus two`.
+
+The concept name is 'a plus b'. It is a quick way to declare a concept with variable,
+but if someone define another concept
+
+::
+
+    def concept number1 plus number2 where number1,number2 as ...
+
+This will produce another concept (with the same key although). I guess that, at
+some point, Sheerka will be able to detect that the concepts are the same, but
+the name of the concept includes its variables. Which may be annoying in some
+situations.
+
+Plus, it's not possible to define rules precedences in this way. For example,
+
+::
+
+    def concept a plus b as ...
+    def concept a times b as ...
+
+How do you express that multiplications have a higher priority in for example
+:code:`one plus two times three` ?
+
+The only right answer, at least to me, is to implement something that is inspired
+by the BNF definition of a grammar.
+
+So the definition of the concept will look like
+
+::
+
+    def concept term as factor (('+' | '-') term)?
+    def concept factor as number (('*' | '/') factor)?
+    def number where number in ['one', 'two', 'three'] as match(body, 'one', 1, 'two', 2, 'three', 3)
+
+This form seems great, but in the definition of term and factor, there is no more
+room for the real body. ie once the components are recognized, what do we do with them ?
+
+So we can try
+
+::
+
+    def concept factor (('+') factor)* as factor[0] + factor[i]
+    def concept number (('*') number)? as number[0] + number[i]
+    def number where number in ['one', 'two', 'three'] as match(body, 'one', 1, 'two', 2, 'three', 3)
+
+The body is defined, but the name of concept is to complicated ex: factor (('+') factor)*
+It's quite impossible to reference a concept that is defined in this way.
+
+So my last proposal, with marry the two ideas, is to introduce the two keyword 'using' 'bnf'
+
+.. _bnf : https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form
+
+::
+
+    def concept term using bnf factor (('+' | '-') term)? as factor + (or -) term
+    def concept factor using bnf number (('*' | '/') factor)? as number * (or /) factor
+    def number where number in ['one', 'two', 'three'] as match(body, 'one', 1, 'two', 2, 'three', 3)
+
+In my implementation:
+
+* Terminals are between quotes
+* Sequences are separated by whitespaces
+* '|' (vertical bar) is used for alternatives
+
+Like in regular expressions, you will also find
+
+* '*' (star) is used to express zero or many
+* '+' (plus) to express one or many
+* '?' (question mark) to expression zero or one
+
+For those who doesn't know that BNF stands for, please have a look at the bnf_
+wikipedia page.
+
+I guess that I will need a complete chapter to explain how you retrieve what was parsed