Proposal: Add concatMapM function (#2042)

[Chris Smith]

Johan Tibell wrote:
> With this approach I would like some facility (e.g. table) to lookup
> common encodings as the encoding used for a particular datum is not know
> at compile time in many applications (e.g. it's read from a HTTP
> Content-Type header or similar.)
> 
>> lookupEncoding :: String -> Maybe Encoding

In fact, there is probably use for several such lookup functions: one for 
each naming scheme for encodings (and there are several of those, largely 
overlapping).  This lookupEncoding function can be implemented after the 
fact -- perhaps in a library dedicated to dealing with MIME content -- 
and it may look something like:

lookupEncoding "ISO-8859-1" = Just iso8859_1
lookupEncoding "ISO-8859-2" = Just iso8859_2
...
lookupEncoding _            = Nothing

And voila!  You've got yourself a way to look up text encodings by name.

All this praise of Java makes me nervous, because as a programmer 
interface, Java is *wrong* about text encodings.  Its standard library 
treats strings, mainly, as the right type for talking about text 
encodings; and they are not!  It keeps one global name-to-encoding 
mapping, assigns each encoding a canonical "Java name", which is 
sometimes invented out of thin air.  The compiler has no list of 
encodings that will be available, so it doesn't complain if you hard-code 
a misspelled encoding name into your program.  This stuff is *really* 
*bad*; it's part of why using Java is a chore rather than a joy.  
Obviously, I'd like to see Haskell avoid that route.

I'm not saying anyone proposed going in that direction; but I got the 
sense that we may be wandering dangerously close.  This is no less 
fundamental than a question of whether we use language features, or fear 
them because we fear committment.  As Simon mentioned, perhaps there are 
more things that need to happen to make the language features to make 
them more compatibility-friendly; but we should make a concerted effort 
to dive right in and use language features for their intended purpose 
rather than timidly hang around the outside fringes.

-- 
Chris Smith