One common oversight in Haskell compilers is failing to intern identifiers
using Ints and failing to prefer IntMaps and IntSets. The PureScript compiler,
for instance, uses Maps as of writing.
Monads for effects are familiar to the Haskell programmer; they were introduced by Wadler's "Monads for functional programming" and are the accepted way to work with side effects in a lazy language.
When writing a compiler, one typically annotates the syntax tree in various phases. What the nodes are annotated with will vary over the course of the program; one might add type information during a type synthesis phase.
I stumbled across a comment by András Kovács on compiler performance, which brought up some of the difficulties writing adequate compilers in lower-level languages such as C++ or Rust.
I have seen "strict data structures, lazy functions" bandied about among Haskellers. This is bad advice. Preferable is "know what you are doing."
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