Understanding the $
The dollar (
$) is a mysterious thing. It doesn't really do anything, but is super useful. It's easy to get it mixed up with other operators in Tidal, for example
#, because in a way they both 'join things together'. But what is
$ is used a lot in Haskell (the language which Tidal lives inside). Like a lot of things in Haskell,
$ is a function. Like all operators (e.g.
+), it has two inputs - the left side, and the right side, and has one output. The left input must be a function, and all that
$ does is pass what's on the right hand side, and give it to that function.
In other words, in this expression:
rev $ "1 2 3"
... the dollar takes
"1 2 3" and passes it to the function
rev. So actually the above is the same as this:
rev "1 2 3"
So if we can do without it, why is it useful? Lets look at a slightly more complex example:
fast 2 $ rev "1 2 3"
Here the dollar takes care of passing
rev "1 2 3" to
fast 2. If we missed it out, then we'd get an error.
-- this gives an error! fast 2 rev "1 2 3"
That's because the computer will first read
fast 2, then
rev, and try to treat
rev as a pattern to be speeded up. But on its own,
rev isn't a pattern, but a function for transforming pattern.
To avoid this error, we could use parenthesis:
fast 2 (rev "1 2 3")
Here the brackets make sure
rev "1 2 3" is calculated first, before it is passed as a pattern to
$ and parenthesis can be used to control which code is calculated first. The
$ is often used to avoid having to match opening and closing brackets, but sometimes parenthesis makes more sense.
Note that you can't really use
$ with operators. For example:
-- this doesn't work either! 4 * $ 2 + 3 -- but this does 4 * (2 + 3)
$ is used to join a parameter (on the right) with a function (on the left).
# (and all its friends
|*|, etc) are used to combine a pattern on the right with a pattern on the left.