Lambda the Ultimate

Are there any special programming language features ("superpowers") that are made possible by specifically exploting a tracing JIT compiler?

It seems to me like tracing JITs are often developed for existing languages (JavaScript, Python, Lua, etc) and tend to focus on efficiently compiling the existing code bases that are written with their traditional programming idioms. I am interested in the opposite approach: what programming idioms and language extensions can help programmers to make the most of their tracing JIT compiler?

I can offer one candidate. Tracing JIT compilers can choose to specialize machine code based on the exact values of parameters that are only available at runtime, and the language can facilitate "hinting" whether a certain variable should be specialized on its value (e.g. generate machine code for the specific case that i = 42) or on its type (e.g. generate machine code for the general case that i is an integer.) Often a certain operations can be compiled much more efficiently when a parameter is constant, for example the size argument to a memcpy or memcmp, and so a potential "superpower" is to apply these optimizations automatically without having to statically generate code that anticipates each possible value.

Is that a legitimate superpower? what are some others? what tricks can tracing JIT users exploit to leave C/C++ hackers eating their dust? :-)

A concise review by Simon Thompson of the second edition of Graham Hutton's Programming in Haskell. The first edition was published in 2007, but chapters were written earlier, and the review focuses on how the language has changed since then, embracing the "categorical / algebraic approach more fully".