Improve diff for un--explained assertion failure

[sjakobi]

For the code below, dhall --file fibs.dhall reports

Use "dhall --explain" for detailed errors

Error: Assertion failed

- [ … ]
+ [ … ]


16│                assert : fibs 10 ≡ [ 0, 1, 2, 3, 5, 8, 13, 21, 34, 55 ]
17│ 

(stdin):16:16

Only when I turn on --explain, dhall shows me the not-so-large detailed diff:

You tried to assert that this expression:                                       
                                                                                
↳ [ 0, 1, 1, 2, 3, 5, 8, 13, 21, 34 ]
                                                                                
... is the same as this other expression:                                       
                                                                                
↳ [ 0, 1, 2, 3, 5, 8, 13, 21, 34, 55 ]
                                                                                
... but they differ

(Aside: Instead of reporting the error location as (stdin):16:16, dhall should include the filename fibs.dhall.)

fibs.dhall:

let Prelude =
      https://raw.githubusercontent.com/dhall-lang/dhall-lang/db16cb26a03130f81c4099c3708d6a18db66defc/Prelude/package.dhall

let fibs =
        λ(n : Natural)
      → let Fib = { cur : Natural, next : Natural }
        
        let succ = λ(x : Fib) → { cur = x.next, next = x.cur + x.next }
        
        in  Prelude.List.map
              Fib
              Natural
              (λ(x : Fib) → x.cur)
              (Prelude.List.iterate n Fib succ { cur = 0, next = 1 })

let testFibs = assert : fibs 10 ≡ [ 0, 1, 2, 3, 5, 8, 13, 21, 34, 55 ]

in  fibs

[Gabriella439]

Fix is up here: #1213