[tel]

The more constrained your theory is, the fewer models you have of it and also the more structure you can exploit.

Monads, I think, offer enough structure in that we can exploit things like monad composition (as fraught as it is), monadic do/for syntax, and abstracting out "traversals" (over data structures most concretely, but also other sorts of traversals) with monadic accumulators.

There's at least one other practical advantage as well, that of "chunking".

A chess master is more capable of quickly memorizing realistic board states than an amateur (and equally good at memorizing randomized board states). When we have a grasp of relevant, powerful structures underlying our world, we can "chunk" along them to reason more quickly. People familiar with monads often can hand-wave a set of unknowns in a problem by recognizing it to be a monad-shaped problem that can be independently solved later.

[ryandv]

> There's at least one other practical advantage as well, that of "chunking".

> When we have a grasp of relevant, powerful structures underlying our world, we can "chunk" along them to reason more quickly.

This is one thing I've observed about Haskell vs. other languages: it more readily gives names and abstractions to even the minutest and most trivial patterns in software, so that seemingly novel problems can be quickly pattern matched and catalogued against a structure that has almost certainly been seen before.

One example: I want to run two (monadic) computations, and then somehow combine together their results (with some binary operation). Such a trivial and fundamental mode of composition, that seems to lack a name in almost every other programming language. Haskell has a name for this mode of composition, and it's called liftM2.

Never again will you have to re-write this pattern for yourself, leaving yourself open to error, now that you have this new concept in your vocabulary. Other languages will happily let you reinvent the wheel for the umpteenth time, or invent idiosyncratic patterns and structures without realizing that they are just particular applications of an already well-studied and well-worn concept.

[maleldil]

Note that this is general enough that you don't need a Monad for this. Applicative is enough (liftA2).