[Haskell-cafe] referential transparency? (for fixity of local operators)
Clark Gaebel
cgaebel at uwaterloo.ca
Fri Oct 5 14:50:32 CEST 2012
Compile with -Wall and the flaw becomes obvious:
<interactive>:2:5:
Warning: This binding for `+' shadows the existing binding
imported from `Prelude' (and originally defined in `GHC.Num')
<interactive>:2:9:
Warning: This binding for `*' shadows the existing binding
imported from `Prelude' (and originally defined in `GHC.Num')
<interactive>:2:16:
Warning: Defaulting the following constraint(s) to type `Integer'
(Num a0) arising from the literal `1'
In the first argument of `(+)', namely `1'
In the first argument of `(*)', namely `1 + 2'
In the expression: 1 + 2 * 3
<interactive>:2:16:
Warning: Defaulting the following constraint(s) to type `Integer'
(Num a0) arising from the literal `1' at <interactive>:2:16
(Show a0) arising from a use of `print' at
<interactive>:2:1-34
In the first argument of `(+)', namely `1'
In the first argument of `(*)', namely `1 + 2'
In the expression: 1 + 2 * 3
Shadowing is bad, and tends (as in this case) to be confusing.
- Clark
On Fri, Oct 5, 2012 at 7:22 AM, Roman Cheplyaka <roma at ro-che.info> wrote:
> * Johannes Waldmann <waldmann at imn.htwk-leipzig.de> [2012-10-05
> 11:11:48+0000]
> > I was really surprised at the following:
> >
> > *Main> 1 + 2 * 3
> > 7
> >
> > *Main> ( \ (+) (*) -> 1 + 2 * 3 ) (+) (*)
> > 9
> >
> > because I was somehow assuming that either
> >
> > a) the Prelude fixities of the operators are kept
> > b) or they are undefined, so the parser rejects.
> >
> > but the Haskell standard says "Any operator lacking a fixity declaration
> > is assumed to be infixl 9". This really should be "infix 9"?
>
> This behaviour is really handy when you use functions as operators
> (using backticks notation). They typically lack infix annotations, but
> having to put parentheses would be very annoying.
>
> Roman
>
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