Haskell supports user-defined operators (made up of symbols) and also lets you use functions in infix position by surrounding the name in backticks, e.g.

    2 `plus` 3

rather than

    plus 2 3

Similarly, Agda has a well-typed mixfix operator syntax – you define a function like (_foo_bar_baz) and can automatically write "X foo Y bar Z baz". It does mean that the operator parser has to be extensible at runtime, but it's not a huge cost for a dependently-typed language.