Haskell 98 Report

[Zhanyong Wan]

Hello Simon,

Looking at the definition for deleteBy:

  deleteBy                :: (x -> a -> Bool) -> x -> [a] -> [a]
  deleteBy eq x []        = []
  deleteBy eq x (y:ys)    = if x `eq` y then ys else
                              y : deleteBy eq x ys

I can't help wondering why it isn't

  deleteBy'          :: (a -> Bool) -> [a] -> [a]
  deleteBy' f []     = []
  deleteBy' f (y:ys) = if f y then ys else
                         y : deleteBy' f ys

The point is that in the definition of deleteBy, all references to eq
and x are in the form (eq x), and hence the two parameters can be
combined.  Is there a reason that the current design was favored when
Prelude was designed?  Thanks.

- Zhanyong

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