Re: [Haskell-cafe] Better writing about Haskell through multi-metaphor learning

On 20 Sep 2021, at 2:30 am, Joachim Durchholz wrote:

The idea is that a monad is a tool to construct a pipeline inside a Haskell program. I think. It isn't my metaphor either, I have seen various people in this thread mention it in passing.

A "pipeline" is not necessarily a *static* pipeline. Even in "bash" we have conditional branching: $ seq 1 99 | while read i; do if (( i % 10 == 1 )); then seq $i | grep 0 | fmt; fi; done 10 10 20 10 20 30 10 20 30 40 10 20 30 40 50 10 20 30 40 50 60 10 20 30 40 50 60 70 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 90 So IO and State and Maybe and Either all fit. As noted a few times in this thread List takes multiple paths. The "non-determinism" (something that not always explained well, since taking all paths is still rather deterministic) extends the simple pipeline model. The computation is still sequenced, it is just that a given stage can invoke its "continuation" multiple times. An intersting minimal model for the list monads is "jq", which is a small functional language for manipulating JSON values. It is basically the List monad for JSON, but both evaluation and function composition are written as simple-looking pipelines: value | function1 | function2 ... But more advanced "jq" users know that a value or function can be a stream (generator) of values: (value1, value2, ...) | function1 | function2 ... $ jq -n '(1,2,3,4) | select(. % 2 == 0) | range(.) | .+100' 100 101 100 101 102 103 So this is isomorphic to Haskell's List monad. And yet, in part because typically there's only one value on the left, and only one output on the right, the naive "pipeline" model is a fairly good fit if, as needed, one also admits "non-determinism". Where pipelines really don't appear to me to be an adequate mental model is "Cont". Reifying continuations feels very different from even a non-deterministic pipeline. Even "programmable semicolon" likely does not yield much intuition about "Cont". But, to first approximation, and without suffering any harm thereby, pipelines have worked well for me (and I expect others). I know that's not the whole/real story, I don't need the model as a crutch, but it works well enough, often enough to remain a useful way of reasoning about the many situations in which it is applicable. -- Viktor.