了解 $

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 $, exactly?

The $ 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 fast 2.

So, both $ 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 use $ with operators. For example:

-- this doesn't work either!
4 * $ 2 + 3
-- but this does
4 * (2 + 3)

Comparing $ and #

So, $ 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.