One can define a Num instance in Haskell for ASTs of expressions, viz.
Haskell programmers have an undue reputation for writing obfuscated code, perhaps due to the fondness for pointfree code and higher-order functions. However the commitment to terseness is dubious at best; syntax is rarely exploited.
"Typing Haskell in Haskell" makes the implementation of type systems concrete for programmers; recent developments in type theory have much to offer but are not developed to this depth even in theory. Row types are particularly juicy because one does not lose type inference; we can use the exact same unification approach and need not resort to ordered contexts or focalization.
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.
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