{"id":148,"date":"2009-03-09T06:21:03","date_gmt":"2009-03-09T05:21:03","guid":{"rendered":"http:\/\/fiber-space.de\/wordpress\/?p=148"},"modified":"2009-03-09T06:22:33","modified_gmt":"2009-03-09T05:22:33","slug":"the-thunk_stmt-and-programmable-semantics-in-python","status":"publish","type":"post","link":"http:\/\/fiber-space.de\/wordpress\/2009\/03\/09\/the-thunk_stmt-and-programmable-semantics-in-python\/","title":{"rendered":"The thunk_stmt and programmable semantics In Python"},"content":{"rendered":"

Seems as if there was just a single direction of syntactical improvement of Python these days which is the introduction of Ruby style blocks. Tav suggests<\/a> to reuse Pythons with-statement:<\/p>\n\n

with_stmt: \"with\" expression ([\"as\" target] | [\"do\" [parameter_list]]) \":\" suite<\/pre>\n
Guido responds promptly and rejects the semantic dispersion of the with-statement. No meaning changer by using just a magical `do` keyword. In Tavs vision the do-branch of the with-statement is turned into an anonymous closure that is called by a special `__do__` function. The `__do__` shall also be customizable which finally leads to DSLs:<\/p>\n
But more enterprising __do__ functions could do interesting things like rewriting the AST of the BLOCK_CODE. Given that this is already done by many many Python libraries, it’s not much of a stretch of the imagination. The benefit? Super nice Pythonic DSLs!<\/p><\/blockquote>\n\nWell it is quite easy to put this idea upside down. Instead of re-purposing a special purpose statement like `with` one can simply turn each expression and “small statement” e.g. raise, assert, return, assignment etc. into a compound statement using following definition<\/p>\n\n
thunk_stmt: small_stmt ':' suite<\/pre>\n
One has to elaborate on it a little and change rules of Pythons existing grammar for LL(1) conformance<\/p>\n\n
stmt: compound_stmt\r\nsingle_input: NEWLINE | compound_stmt NEWLINE\r\ncompound_stmt: ( if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | \r\n                 funcdef | classdef | thunk_stmt )\r\nthunk_stmt: small_stmt ( ':' suite | (';' small_stmt)* [';'] NEWLINE )<\/pre>\n
Here the `thunk_stmt` is a fusion of the definition stated above and the `simple_stmt` of Pythons grammar. This definition doesn’t lead to programmable syntax<\/em> but programmable semantics which is what Tav intends to support with a customizable `__do__`.<\/p>\n
An obvious use case would be property definitions:<\/p>\n\n
x = property():\r\n    def get(self):\r\n        return self._x\r\n\r\n    def set(self, value):\r\n        self._x = value<\/pre>\n
It simply appends a block of code to a valid assignment `x = property()`.<\/p>\n
Same goes with<\/p>\n\n
employees.select.do(emp)<\/pre>\n
which might be extended to<\/p>\n\n
employees.select.do(emp):\r\n    if emp.salary > developer.salary:\r\n            return fireEmployee(emp)\r\n    else:\r\n        return extendContract(emp)<\/pre>\n
Without an appended block the expression<\/p>\n\n
employees.select.do(emp)<\/pre>\n
might simply mean<\/p>\n\n
employees.select(lambda emp: emp)<\/pre>\n
It should be clear that Python needs a protocol to support this which means here that a `__special__` function is attached to an object and that for user defined classes this `__special__` function can be overwritten.<\/p>\n","protected":false},"excerpt":{"rendered":"
Seems as if there was just a single direction of syntactical improvement of Python these days which is the introduction of Ruby style blocks. Tav suggests to reuse Pythons with-statement: with_stmt: “with” expression ([“as” target] | [“do” [parameter_list]]) “:” suite … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[5,12,4],"tags":[],"_links":{"self":[{"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/posts\/148"}],"collection":[{"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/comments?post=148"}],"version-history":[{"count":19,"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/posts\/148\/revisions"}],"predecessor-version":[{"id":167,"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/posts\/148\/revisions\/167"}],"wp:attachment":[{"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/media?parent=148"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/categories?post=148"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/fiber-space.de\/wordpress\/wp-json\/wp\/v2\/tags?post=148"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}