rslt ← (fun ##.file) fname              ⍝ Apply function to native file tie.

Native file 'fname' is opened;  the function operand called  with the tie number
as  argument;  and  the file closed again. The result of the derived function is
the result of the operand function.

Technical notes:

The coding is fairly straightforward.

    file←{                  ⍝ Apply operand function to native file tie.
        tie←⍵ ⎕NTIE 0       ⍝ open file.
        rslt←⍺⍺ tie         ⍝ operate on file.
        {rslt}⎕NUNTIE tie   ⍝ close and return result.
    }

SL suggests the following "crazy" alternative coding:

    file←{
        ⎕ml←1
        ⊃⊢∘⎕nuntie\⌽⊢∘⍺⍺\2⍴⍵ ⎕ntie 0
    }

Local variables are avoided by duplicating a value to produce a pair and operat-
ing on each item of the pair in turn. The sequence of events could be represent-
ed graphically by the following "railway diagram":

     ⊃ ⊢∘⎕nuntie\⌽ ⊢∘⍺⍺\2⍴⍵ ⎕ntie 0         expression
     ↓   ↓       ↓   ↓   ↓  ↓
   ──┐├─[⎕nuntie]─┐─[⍺⍺]─┬─[⎕ntie]──        railway diagram
     └───────────┘└──────┘
      ↑  ↑        ↑  ↑   ↑  ↑
      │  │        │  │   │  └───⍵ ⎕ntie 0── opens file and returns tie number.
      │  │        │  │   └────2⍴─────────── duplicates tie number into a pair.
      │  │        │  └───⊢∘⍺⍺\───────────── applies ⍺⍺ to second item of pair.
      │  │        └───⌽──────────────────── swaps the items of the pair.
      │  └───⊢∘⎕nuntie\──────────────────── unties pair's new second item.
      └───⊃──────────────────────────────── extracts pair's new first item.

Example:

      ⎕nsize file ⎕wsid,'.dws'      ⍝ size of saved version of this workspace.
217608

See also: getfile xtabs

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