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danielx
*nix forums beginner
Joined: 18 Jul 2006
Posts: 8
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 Posted: Thu Jul 20, 2006 9:02 pm Post subject:
Re: question about what lamda does
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Bruno Desthuilliers wrote:
| Quote: | danielx wrote:
(snip)
Python's lambda really can't be as powerful as Lisp's because Python
does not have expressions that do case analysis (this is not lambda's
fault, of course  . The reason is that you really want to put each
case on its own set of lines. This enhances readability at the expense
of terseness. Since Python's statements are terminated by a newline, it
would be rather awkward to have a kind of expression where good style
calls for it to be spread out accross multiple lines.
You can try to simulate these kinds expressions using into a list or
dictionary, but this becomes rather messy. I think the only way to get
this done properly is to use eval. For example:
def recursiveFunction(args):
... # do stuff...
choices = { True:"0", False:"recurisveFunction(newArgs)" }
return eval( choices[predicate] )
Why do you want to use eval here ?
The reason that you need eval is that you want to prevent any cases
from being executed until you decide which one you want.
What about:
def recursiveFunction(args):
... # do stuff...
... # that defines 'newArgs' and 'predicate' of course ...
return (recursiveFunction, lambda x: 0)[predicate](newArgs)
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Sure, that works, but don't take things so literally. For instance, if
you have a bunch of cases, you might not way to apply the same set of
arguments to all of them.
Also, let's not get distracted from the main point about how doing case
analysis in an expression is ugly, making lambda's weaker in Python
than in the language which inspired them.
| Quote: |
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom.gro'.split('@')])" |
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bruno modulix
*nix forums Guru
Joined: 21 Feb 2005
Posts: 819
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| Posted: Fri Jul 21, 2006 1:03 pm Post subject:
Re: question about what lamda does
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danielx wrote:
| Quote: | Bruno Desthuilliers wrote:
danielx wrote:
(snip)
Python's lambda really can't be as powerful as Lisp's because Python
does not have expressions that do case analysis (this is not lambda's
fault, of course . The reason is that you really want to put each
case on its own set of lines.
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An expression can span several lines.
| Quote: | This enhances readability at the expense
of terseness. Since Python's statements are terminated by a newline,
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or by a ';'
| Quote: | it
would be rather awkward to have a kind of expression where good style
calls for it to be spread out accross multiple lines.
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I must be pretty dumb, but I don't see how this relate to the problem of
case analysis in lambda expressions ?
| Quote: | You can try to simulate these kinds expressions using into a list or
dictionary, but this becomes rather messy. I think the only way to get
this done properly is to use eval. For example:
def recursiveFunction(args):
... # do stuff...
choices = { True:"0", False:"recurisveFunction(newArgs)" }
return eval( choices[predicate] )
Why do you want to use eval here ?
The reason that you need eval is that you want to prevent any cases
from being executed until you decide which one you want.
What about:
def recursiveFunction(args):
... # do stuff...
... # that defines 'newArgs' and 'predicate' of course ...
return (recursiveFunction, lambda x: 0)[predicate](newArgs)
Sure, that works, but don't take things so literally.
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Sorry for being pragmatic !-)
| Quote: | For instance, if
you have a bunch of cases, you might not way to apply the same set of
arguments to all of them.
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return {
'case1' : lambda: someFunc(args1),
'case2' : lambda: someFunc(args2),
'case3' : lambda: someOtherFunc(args1, arg42),
}.get(predicate, lambda: 0)()
Still no need for eval()...
Now of course there are limits to the exercice, and we're still far away
from ML-like pattern matching or Lisp 'case' forms. As you noted, Python
is a statement-based language, not an expression-based one like Lisp.
This makes a definitive difference.
| Quote: | Also, let's not get distracted from the main point about how doing case
analysis in an expression is ugly,
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Ugliness is in the eyes of the beholder <wink>
| Quote: | making lambda's weaker in Python
than in the language which inspired them.
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The fact is that Python "lambdas" are *not* Lisp lambdas. Python
"lambdas" are mostly a handy trick to turn a *simple* expression into a
closure - and definitively not a basic building block of the language.
Daniel, I of course do agree that Python lambdas are nothing near Lisp
lambdas - FWIW, Python is not Lisp neither -, but that looks like an
apple and banana comparison to me... IMHO, the most obvious problem with
Python lambdas is the word "lambda" !-)
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom.gro'.split('@')])" |
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