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Created July 3, 2011 08:58
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Haskell operator section
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ablog.md

In this wonderful tutorial "Write Yourself a Scheme in 48 Hours", this line puzzles me for a while:

apply :: String -> [LispVal] -> LispVal
apply func args = maybe (Bool False) ($ args) $ lookup func primitives

What does ($ args) really mean? Turns out it is called operator section, a special syntax which LYSH fails to mention. This thread did a great job on explaining it:

http://www.haskell.org/pipermail/beginners/2009-January/000776.html

And the definition of it is here:

http://www.haskell.org/onlinereport/exps.html#sections

and here:

http://www.haskell.org/haskellwiki/Section_of_an_infix_operator

Also be careful that - operator cannot do a right section, use Prelude subtract instead.

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