Comments on: Norvig’s spell checker and idiomatic Haskell http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/ Bryan O'Sullivan's blog Wed, 08 Feb 2012 06:41:38 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: Daniel http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46131 Daniel Tue, 15 May 2007 19:29:57 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46131 "... not implementing the approach that would be best suited to Haskell (dynamic programming)." Could you be convinced to do that? Would *love* to see that explained as nicely as you have explained this implementation. I really enjoyed your article. “… not implementing the approach that would be best suited to Haskell (dynamic programming).”

Could you be convinced to do that? Would *love* to see that explained as nicely as you have explained this implementation.

I really enjoyed your article.

]]> By: Miles Gould http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46105 Miles Gould Tue, 15 May 2007 16:03:55 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46105 Ah, OK. Though I think it's only messy because you have to define updateWith() yourself (and because defaultdicts aren't available on the Python interpreter I have to hand). If you want to do this kind of thing with folds rather than loops, then write updateWith once, stick it in a module somewhere, and import it when you need it :-) Ah, OK. Though I think it’s only messy because you have to define updateWith() yourself (and because defaultdicts aren’t available on the Python interpreter I have to hand). If you want to do this kind of thing with folds rather than loops, then write updateWith once, stick it in a module somewhere, and import it when you need it :-)

]]> By: Bryan O'Sullivan http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46097 Bryan O'Sullivan Tue, 15 May 2007 15:40:24 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46097 Miles, I know about reduce, but as I pointed out and you confirm, the code you end up writing to use it to update a dict is messy. Miles, I know about reduce, but as I pointed out and you confirm, the code you end up writing to use it to update a dict is messy.

]]> By: Miles Gould http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46094 Miles Gould Tue, 15 May 2007 15:22:28 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46094 <i>The Python code is a fold, too; there’s just not a clean way to write a fold that augments a dict in Python. So instead we have a loop that somewhat obfuscates the fact that we’re folding.</i> Huh? The reduce() function has been part of Python for as long as I've been aware of the language, and the following works fine for me: def updateWith(f, default): def curried(dict,index): if index in dict: dict[index] = f(dict[index]) else: dict[index] = default return dict return curried print reduce(updateWith(lambda x: x + 1, 1), [1,2,3,4,5,1,3,7,0], {} This prints <code>{0: 1, 1: 2, 2: 1, 3: 2, 4: 1, 5: 1, 7: 1}</code> as expected. It would probably be considered poor style, though, and I believe there's an ongoing argument in the Python community as to whether reduce() and map() should be removed from the language entirely: I guess they don't buy your argument that code using map and fold is clearer than the equivalent code using loops :-) Other than that, great article! The Python code is a fold, too; there’s just not a clean way to write a fold that augments a dict in Python. So instead we have a loop that somewhat obfuscates the fact that we’re folding.

Huh? The reduce() function has been part of Python for as long as I’ve been aware of the language, and the following works fine for me:
def updateWith(f, default):
def curried(dict,index):
if index in dict:
dict[index] = f(dict[index])
else: dict[index] = default
return dict
return curried

print reduce(updateWith(lambda x: x + 1, 1), [1,2,3,4,5,1,3,7,0], {} This prints {0: 1, 1: 2, 2: 1, 3: 2, 4: 1, 5: 1, 7: 1} as expected.

It would probably be considered poor style, though, and I believe there’s an ongoing argument in the Python community as to whether reduce() and map() should be removed from the language entirely: I guess they don’t buy your argument that code using map and fold is clearer than the equivalent code using loops :-)

Other than that, great article!

]]> By: Harald Korneliussen http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46049 Harald Korneliussen Tue, 15 May 2007 07:33:56 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46049 It does that. With optimisation, it seems to me foldl never causes trouble. Why shouldn't the compiler be better at strictness analysis than me? It does that. With optimisation, it seems to me foldl never causes trouble. Why shouldn’t the compiler be better at strictness analysis than me?

]]> By: Bryan O'Sullivan http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46024 Bryan O'Sullivan Tue, 15 May 2007 04:03:43 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46024 I've fixed the article. Oops! In my defence, I had substituted <code>foldl</code> for <code>foldl'</code> and found no difference in performance or memory use, and was even looking at profiler output and seeing a nice slow heap growth instead of an explosion. I conclude that the compiler was being cleverer than I was. I’ve fixed the article. Oops! In my defence, I had substituted foldl for foldl' and found no difference in performance or memory use, and was even looking at profiler output and seeing a nice slow heap growth instead of an explosion. I conclude that the compiler was being cleverer than I was.

]]> By: Derek Elkins http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46022 Derek Elkins Tue, 15 May 2007 03:42:38 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46022 petekaz is correct. As illustrated on http://www.haskell.org/hawiki/StackOverflow foldl will accumulate thunks and lead to a stack overflow in just about all cases. foldl is pretty much useless. foldl' should be what is used if you have a strict function and foldr otherwise. petekaz is correct. As illustrated on http://www.haskell.org/hawiki/StackOverflow foldl will accumulate thunks and lead to a stack overflow in just about all cases. foldl is pretty much useless. foldl’ should be what is used if you have a strict function and foldr otherwise.

]]> By: petekaz http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46019 petekaz Tue, 15 May 2007 03:36:28 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46019 For more discussion on the subject, see this thread on the cafe mailing list: http://thread.gmane.org/gmane.comp.lang.haskell.cafe/21780 For more discussion on the subject, see this thread on the cafe mailing list:

http://thread.gmane.org/gmane.comp.lang.haskell.cafe/21780

]]> By: Daniel Lyons http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46013 Daniel Lyons Tue, 15 May 2007 02:34:31 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46013 Wow! I worked on this problem a month or two ago when Norvig's spellchecker hit Reddit, but my Haskell implementation was ugly, inefficient and probably incorrect. Your elucidation clarifies all of the problems I ran into and results in beautiful, concise code. This is a service to the world and I thank you! Wow!

I worked on this problem a month or two ago when Norvig’s spellchecker hit Reddit, but my Haskell implementation was ugly, inefficient and probably incorrect. Your elucidation clarifies all of the problems I ran into and results in beautiful, concise code. This is a service to the world and I thank you!

]]> By: Pied http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/comment-page-1/#comment-46011 Pied Tue, 15 May 2007 02:23:04 +0000 http://www.serpentine.com/blog/2007/05/14/norvigs-spell-checker-and-idiomatic-haskell/#comment-46011 Wow ! I'm not sure whether this was intended to be used as a tutorial, but it really is the best example of Haskell I have ever read ! I wish you had written it a year or two earlier... I am to show your article to many people I know who have been frightened with the usual Haskell articles... P! Wow !
I’m not sure whether this was intended to be used as a tutorial, but it really is the best example of Haskell I have ever read !
I wish you had written it a year or two earlier…

I am to show your article to many people I know who have been frightened with the usual Haskell articles…

P!

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