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POpython: mutating `globals` to dynamically put things in scope
 

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    <p>how terrible an idea is this? class <code>monad</code> implements the <code>with</code> interface to put things in and out of scope, so i can write a library of generic functions like m_chain who refer to functions <code>unit</code> and <code>bind</code> who can have an implementation put in at runtime. (It doesn't matter what all this code does or if it's a good idea.)</p> <p>other ideas i tried all revolved around passing around a structure containing unit/bind as an argument or a kwarg, or putting m_chain in a class, implement it in terms of self.unit and self.bind and having derived classes provide them. but it added complexity to the code and syntax and tied unit/bind to the way monads are expressed in python. using scope for this just feels so much nicer.</p> <pre><code>class monad: """Effectively, put the monad definition in lexical scope. Can't modify the execution environment `globals()` directly, because after globals().clear() you can't do anything. """ def __init__(self, monad): self.monad = monad self.oldglobals = {} def __enter__(self): for k in self.monad: if k in globals(): self.oldglobals[k]=globals()[k] globals()[k]=self.monad[k] def __exit__(self, type, value, traceback): """careful to distinguish between None and undefined. remove the values we added, then restore the old value only if it ever existed""" for k in self.monad: del globals()[k] for k in self.oldglobals: globals()[k]=self.oldglobals[k] def m_chain(*fns): """returns a function of one argument which performs the monadic composition of fns""" def m_chain_link(chain_expr, step): return lambda v: bind(chain_expr(v), step) return reduce(m_chain_link, fns, unit) identity_m = { 'bind':lambda v,f:f(v), 'unit':lambda v:v } with monad(identity_m): assert m_chain(lambda x:2*x, lambda x:2*x)(2) == 8 maybe_m = { 'bind':lambda v,f:f(v) if v else None, 'unit':lambda v:v } with monad(maybe_m): assert m_chain(lambda x:2*x, lambda x:2*x)(2) == 8 assert m_chain(lambda x:None, lambda x:2*x)(2) == None </code></pre>
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    1. COSeems legit. I'm interested in what some of the answers will be. I dig the effect, letting you swap in and out of variations of a function on demand. If there's a downside I don't see one. I'd probably be careful to provide default definitions of the functions `m_chains` is going to use, or throw a meaningful exception if they're not set, but otherwise I can see some decent applications for something like this.
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