wdte: add infix notation

[DeedleFake]

In Haskell, a function can be called in an infix position, such as arg1 func arg2, by surrounding the function name with backticks. I'm not a big fan of Haskell's particular syntax for infix notation, but support for some means of providing infix notation would be nice.

Current ideas:

• {arg1 func arg2}, in which all three elements are <single>s. The downside is that an expression like {fib {n - 1} + fib {n - 2}} would be unparsable. {(fib {n - 1}) + (fib {n - 2})} would be fine, though, so it's probably fine, and extreme cases could just stick the arguments in variables first to make it cleaner.
• arg1 {func} arg2, in which all three elements are arbitrary expressions. This seems a bit more awkward to parse and is depressingly similar to the Haskell notation.

[DeedleFake]

After attempting to implement the first variant ({arg1 func arg2}), I have discovered a problem that affects both. The usage of { at the beginning of an expression introduces an ambiguity with switches, as switches start with switch <expr> {. This isn't an issue if the function at the beginning of <expr> is the infix expression, but if an argument is, such as in switch f {1 + 2} {, then the parser can't tell that the first { isn't the beginning of the switches cases.

The only solutions I can think of are either to use a completely different infix notation, or to change the <switch> syntax. I'm leaning towards the former, but I'm not quite sure what to use.