in reply to Currying--useful examples?

Here's the example I usually give in talks etc.. Say you have an HTML editor that has bold, underline and italics. Instead of writing three functions, have the compiler write them for you: 

sub mk_tagger {
    my($tag) = @_;
    return sub { my($txt) = @_; "<$tag>$txt</$tag>" };
}
my ($bold, $underline, $italic) = map { mk_tagger($_) } qw( b u i );

print $bold->("what a " . $underline->("nice") . $italic->(" day"));

[download]

Here's the equivalent Perl 6: 

sub mk_tagger ($tag) {
    -> { "<$tag>$^txt</$tag>" }
}

[download]

The idea is that the mk_taggger function is intentionally designed not to be used directly but to generate tagging functions.

Update: In Haskell and other languages that had autocurrying, you'd not need this explicit distinction. 

tagStr :: String -> String -> String
tagStr tag text = "<" ++ tag ++ ">" ++ text ++ "</" ++ tag ++ ">"

-- now I can use it either way.
tagStr "b" "a bold moose"
underline = tagStr "u"
underline "an underlined moose"

[download]

Replies are listed 'Best First'.
Re^2: Currying--useful examples?
by gaal (Parson) on Jan 10, 2007 at 20:50 UTC
    Of course, to say "<" ++ tag ++ ">" is very ugly, so you'd want to write it as angle tag. angle is a function? How is it implemented? In typical function programming style: 

    angle :: String -> String
angle s = surround "<" ">" s
{- This can, in "points free" style, be written:
angle = surround "<" ">"  -- look ma, no explicit variable!
 -}

surround :: String -> String -> String -> String
surround start end text = start ++ text ++ end

-- The idea is to reuse surround:
quote   = surround "'" "'"
bracket = surround "[" "]"
braces  = surround "{" "}"

... etc.

    [download]
    But this is straying off-topic for this site...
[reply]
[d/l]
[select]

In Section Seekers of Perl Wisdom