[Haskell-cafe] Precedence (Part 2)

Brandon S. Allbery KF8NH allbery at ece.cmu.edu
Thu Apr 3 13:33:41 EDT 2008

On Apr 3, 2008, at 13:13 , PR Stanley wrote:
> Hi folks
> > let f = sum.map (^2).filter even
> > f [1..4]
> 20
> So far so good!
> > sum.map (^2).filter even [1..4]
> returns an error.
> How is it that the composition above as part of a function equation  
> doesn't return any errors while on its own it requires parentheses?

You seem to have expression evaluation confused with cpp/m4 macros.   
The `f' in `f [1..4]' is not expanded as text to `sum.map (^2).filter  
even [1..4]'; it is evaluated as a function whose definition will  
(when needed!  lazy language) eventually turn out to be `sum.map  
(^2).filter even'.

If you really must view it as a text macro substitution, then imagine  
that every macro expansion has a set of parentheses added around it.

brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH

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