It can take some wrangling to avoid space leaks and you often have to be careful about strictness (especially when a small change in the code causes the compiler to miss one of the big optimisations), while languages like Rust and C++ put more control into the hands of the programmer and thus achieve high performance more naturally.
As an example of this, consider the benchmarks game[1]. The Rust and Haskell are on par[2] for the most part.
Except, the Haskell programs are having to pull out all stops to reach this level, they essentially all have a ton of strictness annotations, and are doing manual pointer manipulations and even manual allocations. Of the fastest Haskell solutions (i.e. the ones linked on [2]) I count two that don't use a `Foreign.*` module (pidigits and binary-trees) and only one that uses no !'s (pidigits).
On the other hand, none of the Rust programs use any `unsafe` at all (that is, they are guaranteed to be memory safe, e.g. no risk of out-of-bounds accesses or dangling pointers), and are generally not particularly optimised (e.g. the Rust program that is 85 times slower than the Haskell just appears to have been written without thinking about performance at all... it does a whole formatted printing call for each and every character in the output! The version in-tree[3] is at least 50 times faster and still uses no unsafe code).
(I say this as someone who likes Haskell: I'm the person with the most votes for answers in the [rust] tag on StackOverflow, but I've got even more votes than that for my answers on the [haskell] tag.)
As an example of this, consider the benchmarks game[1]. The Rust and Haskell are on par[2] for the most part.
Except, the Haskell programs are having to pull out all stops to reach this level, they essentially all have a ton of strictness annotations, and are doing manual pointer manipulations and even manual allocations. Of the fastest Haskell solutions (i.e. the ones linked on [2]) I count two that don't use a `Foreign.*` module (pidigits and binary-trees) and only one that uses no !'s (pidigits).
On the other hand, none of the Rust programs use any `unsafe` at all (that is, they are guaranteed to be memory safe, e.g. no risk of out-of-bounds accesses or dangling pointers), and are generally not particularly optimised (e.g. the Rust program that is 85 times slower than the Haskell just appears to have been written without thinking about performance at all... it does a whole formatted printing call for each and every character in the output! The version in-tree[3] is at least 50 times faster and still uses no unsafe code).
(I say this as someone who likes Haskell: I'm the person with the most votes for answers in the [rust] tag on StackOverflow, but I've got even more votes than that for my answers on the [haskell] tag.)
[1]: http://benchmarksgame.alioth.debian.org/
[2]: http://benchmarksgame.alioth.debian.org/u64/benchmark.php?te...
[3]: https://github.com/rust-lang/rust/blob/master/src/test/bench...