Re: [web-devel] xml-types IsString instance for Name causes crashes

On Fri, Jun 10, 2011 at 9:40 PM, John Millikin wrote:

On Fri, Jun 10, 2011 at 07:20, Simon Meier wrote:

...

Note that the OverloadedStringLiterals together with the 'IsString' instances gives you more than just a nice syntactic abbreviation: the defined string literals are also lifted out of their context and defined as CAFs. Therefore, you can share the conversion from String to <your-type> during the whole run of a program. We exploit that in blaze-html to do escaping and UTF-8 encoding only once. Hence, in some contexts the IsString instances are necessary.

I'm afraid I don't understand this; could you explain the difference between these two definitions? Specifically, why would the OverloadedStrings version be more efficient?

------------------------------------------------------- foo :: Name foo = Name "foo" (Just "urn:bar") Nothing -------------------------------------------------------

------------------------------------------------------- {-# LANGUAGE OverloadedStrings #-} foo :: Name foo = "{urn:bar}foo" -------------------------------------------------------

Warning: IANAS[1]. I think these two *would* be identical, the difference would be if you don't declare foo at the top level like this, but instead use it inline twice. In such a case, the OverloadedStrings version would be CAFed, while the direct Name constructor would like the "foo" and "urn:bar" Text values be CAFed but not the Name value itself. Michael [1] I am not a Simon.

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