Two chaining/piping macros, one of them `setf`ing its first
 argument

chain

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# Chain

###### \[in package CHAIN\]
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- [system] **"chain"**
    - Version: 0.0.1
    - Description: Two chaining/piping macros, one of them [`setf`][a138]ing its first
        argument
    - Licence: BSD-3
    - Author: Thomas Bartscher <thomas-bartscher@weltraumschlangen.de>
    - Depends on: metabang-bind, mgl-pax

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- [macro] **=>** *ARGUMENT &BODY BODY*

    Thread a value through a series of transformations, where `%` represents the
    value of the last transformation.
    
    - Arguments
    
      - `argument`: Initial value or a `(:var SYMBOL INITIAL-VALUE)` binding form.
    
      - `body`: Forms to apply sequentially. Each form has `%` bound as an anaphoric
        variable bound to the return value of the previous form in the body.
        Each form may be:
    
        - A regular form
    
        - A `(:var SYMBOL EXPR)` form to bind [`SYMBOL`][e5af] to the result of evaluating
         `EXPR`
    
        - A `(lambda (X) …)` form that is called on the result of the previous form.
    
    - Return value
    
    Returns the value of its last form.
    
    - Examples
    
    ```lisp
    ;; Basic numeric pipeline
    (=> 5
      (* % 2)  ; => 10
      (1+ %))
    => 11
    
    ;; lambda form usage
    (=> 5
      (lambda (x)  ; => 8
        (+ x 3))
      (* % 2))
    => 16
    
    ;; :var binding
    (=> 5
      (:var y (* % 3))  ; y = 15, % = 15
      (1+ %)            ; % = 16
      (+ y %))          ; y + % = 15 + 16
    => 31
    
    ;; :var in argument position
    (=> (:var a 5)  ; a = 5, % = 5
      (1+ %)        ; % = 6
      (+ a %))      ; a + % = 5 + 6
    => 11
    ```


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- [macro] **SET=>** *ARGUMENT &BODY BODY*

    Thread the initial value of a place through transformations like in macro
    [`=>`][8fe2], and update the place with the final result.
    
    - Arguments
    
      - `argument`: A [`setf`][a138]-able place or `(:var SYMBOL PLACE)` where [`SYMBOL`][e5af] is a
        variable name and `PLACE` is a `setf`-able place. When refering to `PLACE` from
        here on out, the `setf`-able place is meant.
    
      - `body`: Exactly the same as in macro [`=>`][8fe2]. Supports `:var`, `lambda`([`0`][e400] [`1`][5c01]) and `%`
        the same as `=>`.
    
    - Return value
    
      Returns the value of its last form.
    
    - Side effects
    
      Updates `PLACE` to the calculated result.
    
    - Examples
    
    ```lisp
    ;;; Modify a variable
    (let ((x 5))
      (set=> x
        (* % 2)  ; => 10
        (1+ %))  ; => 11
      x)
    => 11
    
    ;; Using :var
    (defstruct box (value 0))
    (let ((b (make-box :value 8)))
      (set=> (:var v (box-value b))
        (+ % 5)   ; => 13
        (* v %))  ; => 8 * 13 = 104
      (box-value b))
    => 104
    ```


  [5c01]: http://www.lispworks.com/documentation/HyperSpec/Body/m_lambda.htm "LAMBDA (MGL-PAX:CLHS MGL-PAX:MACRO)"
  [8fe2]: #CHAIN:%3D%3E%20MGL-PAX:MACRO "CHAIN:=> MGL-PAX:MACRO"
  [a138]: http://www.lispworks.com/documentation/HyperSpec/Body/m_setf_.htm "SETF (MGL-PAX:CLHS MGL-PAX:MACRO)"
  [e400]: http://www.lispworks.com/documentation/HyperSpec/Body/s_lambda.htm '"s_lambda" (MGL-PAX:CLHS MGL-PAX:SECTION)'
  [e5af]: http://www.lispworks.com/documentation/HyperSpec/Body/t_symbol.htm "SYMBOL (MGL-PAX:CLHS CLASS)"


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