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That's what initially drew me to Haskell, and how it seemed closer to math than most other programming languages.

Lazy evaluation brings this to the fore, especially with needing the programmer to make decisions about the sequencing of operations.

Consider these two statements:

  x = a + 5;
  y = b + 3;

So these statements describe the relationship between 'a' and 'x', and 'b' and 'y'. Great. With most programming languages, the programmer also must choose the sequencing of these operations. When they should occur, and in what order. With Haskell, this goes away.

The compiler / runtime system decides when (or if) the values of 'x' and 'y' will be evaluated. With a really, really smart compiler / runtime system, this can lead to great optimizations a C programmer would need to implement by hand when porting code to a new platform with vastly different performance characteristics.

Now... in practice... I found Haskell hard to get comfortable with. I'm quite happy with Rust these days though.