error.tao

import "value.tao"
import "fmt.tao"

data Result A, E =
    | Ok A
    \ Err E

data Never =|

# TODO: Reenable when coherence checker is cleverer
# for A member Result A Never of From A =
#     => from = fn x => Ok x

# TODO: Reenable when coherence checker is cleverer
# for A member A of From (Result A Never) =
#     => from = fn Ok x => x

for A < Show, E < Show member Result A E of Show =
    => show = fn
        | Ok x => x->show
        \ Err e => "error: " ++ e->show

for A < Debug, E < Debug member Result A E of Debug =
    => debug = fn
        | Ok x => "Ok " ++ x->debug
        \ Err e => "Err " ++ e->debug

for A, B, E, e member Result A E of Map A B e =
    => Output = e ~ Result B E
    => map = fn
        | f, Ok a => Ok a->f!
        \ _, Err e => Err e

$[util]
fn map_err A, E, F : (E -> F) -> Result A E -> Result A F =
    | _, Ok x => Ok x
    \ f, Err e => Err e->f