[Haskell-cafe] Pattern matching does not work like this?

Richard O'Keefe ok at cs.otago.ac.nz
Wed Jul 15 21:56:07 EDT 2009

On Jul 15, 2009, at 9:57 PM, Magicloud Magiclouds wrote:

> Hi,
>  I do not notice this before. "fun ([0, 1] ++ xs) = .." in my code
> could not be compiled, parse error.

I apologise to everyone for my previous message in this
thread.  There was a Haskell message in amongst some Erlang
messages, and I thought this was an Erlang problem.

There are some programming languages, including Erlang,
in which <explicit list> ++ <pattern> IS accepted as a
<pattern>.  Haskell is not one of them.  It could be,
and there would be no ambiguity in elementary cases,
and it would no more involve matching against ++ than
n+1 involves matching against +.  Since the pattern on
the left of the ++ is required to be an explicit list,
there is no the slightest question of backtracking or
anything more general than ordinary pattern matching.


The second is in general shorter (though less clear; it is
a real pity that Haskell syntax doesn't include something
like Clean's [head:tail], the inconsistency is irritating).
It's not the general case that this syntax was invented for,
but the case where the list is a string.


One of the arguments advanced against n+k patterns is
"which scope should + be looked up in? what if the + in
the standard prelude is not in scope?"

The same question can be asked about list++tail patterns;
what should it mean if the prelude's ++ was not in scope?

Programmers who have met "abc"++Xs sometimes turn around
and ask for Xs++"abc".  Perhaps it is better to keep the
camel's nose out of the tent.

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