Example: `Result` workflows

Result a e short-circuits on Err.

Goal

Model a computation that can fail with a useful error, while keeping “happy path” code easy to read.

let
  step1 = \x -> if x < 0 then Err "negative" else Ok (x + 1),
  step2 = \x -> Ok (x * 2)
in
  ( bind step2 (bind step1 (Ok 10))
  , bind step2 (bind step1 (Ok (0 - 1)))
  )

What to notice

When step1 returns Err, the second bind is skipped.
This lets you write “happy path” code without deeply nested match.

Step-by-step: name the pipeline

let
  step1 = \x -> if x < 0 then Err "negative" else Ok (x + 1),
  step2 = \x -> Ok (x * 2),
  run = \x -> bind step2 (bind step1 x)
in
  (run (Ok 10), run (Ok (0 - 1)))

`map` vs `bind`

Use map when your function does not fail and does not change the container:

map ((+) 1) (Ok 41)

Use bind when your function returns another Result (and might fail):

bind (\x -> if x < 0 then Err "negative" else Ok x) (Ok 1)

Worked examples

Example: fail `step2` when value is too large

Problem: make the second step return Err above a threshold.

let
  step1 = \x -> if x < 0 then Err "negative" else Ok (x + 1),
  step2 = \x -> if x > 20 then Err "too-large" else Ok (x * 2),
  run = \x -> bind step2 (bind step1 x)
in
  (run (Ok 10), run (Ok 25))

Why this works: bind short-circuits on either error source, including the new step2 condition.

Example: custom error ADT

Problem: replace string errors with structured errors.

type Err = Negative | TooLarge

let
  step1 = \x -> if x < 0 then Err Negative else Ok (x + 1),
  step2 = \x -> if x > 20 then Err TooLarge else Ok (x * 2),
  run = \x -> bind step2 (bind step1 x)
in
  (run (Ok 10), run (Ok 25), run (Ok (0 - 1)))

Why this works: error constructors carry precise machine-readable failure categories.

Example: `and_then` synonym

Problem: define a helper that reads like “then”.

let
  and_then = \mx f -> bind f mx,
  safe_inc = \x -> if x < 0 then Err "negative" else Ok (x + 1)
in
  and_then (Ok 1) safe_inc

Why this works: and_then is just argument-reordered bind, so behavior is identical.

Keyboard shortcuts

Rex