# Higher Order Function Question

Alex Peake <>
Fri May 24 18:03:17 CEST 2002

```Richard,

Thanks you for the list comprehension suggestion. I had not thought of that
(it does not exist in the Lisp world where I come from).

I do use complement a lot, so I think, for me, it is worth the definition --
but perhaps if I re-examine my approach, list comprehensions may obviate the
need.

As for
4> apply({attr, isPk}, [A1]).

as I mentioned, I was just testing for eventual use in filter, and as far as
I know, in filter (and map, etc.) you *must* use the form {attr, isPk}. I
always get errors when I try attr:isPk within map, filter etc.

150> lists:map(attr:varName, rel:attrList(rel:findNamed(ng,"Member"))).

Thanks,

Alex

> -----Original Message-----
> From:
> [mailto:]On Behalf Of Richard Carlsson
> Sent: Friday, May 24, 2002 2:23 AM
> To:
> Cc:
> Subject: Re: Higher Order Function Question
>
>
>
> (Sorry for the previous one - I hit "send" by mistake.)
>
> A list comprehension is a bit easier to read, and executes faster
> (becomes a local recursive function):
>
> 	[X || X <- SomeList, not attr:isPk(X)]
>
> > 	lists:filter(fun (Attr) -> not attr:isPk(Attr) end, SomeList)
>
> Yes, this is the way to do it if you prefer to use lists:filter.
> And there is not much of a point in defining a "complement" function
> unless you really need it often in your program.
>
> > > 4> apply({attr, isPk}, [A1]).
>
> Please forget that this form of function call ever existed. Use
>
> 	F(...)
>
> where F = fun ..., or
>
> 	M:F(...)
>
> where M and F are atoms.
>
> Only use apply(M, F, [...]) when you really, really cannot find an
> alternative.
>
> 	/Richard
>
>
>
> On Fri, 24 May 2002, Raimo Niskanen wrote:
>
> > I cannot come up with anything better than:
> >
> > 	lists:filter(fun (Attr) -> not attr:isPk(Attr) end, SomeList)
> >
> > or
> >
> > 	complement(Fun) -> fun (Attr) -> not Fun(Attr) end.
> >
> > 	lists:filter(complement({attr, isPk}), SomeList)
> > 	% or
> > 	lists:filter(complement(fun isPk/1), SomeList) % within
> module 'attr'
> > 	% or
> > 	lists:filter(complement(fun(Attr) -> attr:isPk(Attr) end,
> SomeList) %
> > Very pointless with the complement() function
> >
> > / Raimo Niskanen, Erlang/OTP, Ericsson AB
> >
> >
> >
> >
> > Alex Peake wrote:
> > >
> > > Can someone help me with this (no doubt simple) problem.
> > >
> > > I try:
> > >
> > > 1> attr:isPk(Attr).
> > > false
> > >
> > > 2> not attr:isPk(Attr).
> > > true
> > >
> > > 3> not(attr:isPk(Attr)).
> > > true
> > >
> > > but cannot find a permutation/combination of:
> > >
> > > 4> apply({attr, isPk}, [A1]).
> > > false
> > >
> > > that can include "not" and work.
> > >
> > > Actually, I am trying to lists:filter( ...not isPk... SomeList).
> > >
> > > The simple is to define notIsPk(Attr), or perhaps
> lists:filter( fun(Attr) ->
> > > .. end  but they are not an abstraction of "complement".
> > >
> > > Perhaps I should define a function complement(Fn) that
> returns a function
> > > that is the complement of the function passed?
> > >
> > > Perhaps there is already an Erlang way to do this (I hope).
> > >
> > > Alex
> >
>
> Richard Carlsson ()   (This space intentionally
> left blank.)
> E-mail: 	WWW: http://www.csd.uu.se/~richardc/
>  "Having users is like optimization: the wise course is to delay it."
>    -- Paul Graham
>
>

```