CURRY-COMPOSE-READER-MACROS

Reader macros for concise expression of function partial application and composition.

These reader macros expand into the curry, rcurry and compose functions from the Alexandria library. The contents of curly brackets are curried and the contents of square brackets are composed. The _ symbol inside curly brackets changes the order of arguments with rcurry.

The following examples demonstrate usage.

;; partial application `curry'
(mapcar {+ 1} '(1 2 3 4)) ; => (2 3 4 5)

;; alternate order of arguments `rcurry'
(mapcar {- _ 1} '(1 2 3 4)) ; => (0 1 2 3)

;; function composition
(mapcar [#'list {* 2}] '(1 2 3 4)) ; => ((2) (4) (6) (8))

If the left curly brace is followed by a non-negative integer, the integer defines the number of additional parameters the curried function expects. This allows argument number mismatch to be detected statically and may cause more efficient code to be generated.

;; The + will be on two arguments, one of which
;; is provided
(mapcar {1 + 5} '(1 2 3 4)) ; ==> (6 7 8 9)

Additionally, special brackets may be used to split arguments amongst a list of functions and collect the results. The first element of the «»-delimited list is the "join" function. Incoming arguments are split out to the remaining functions in the «»-delimited list, and their results are then passed to the join function. (Emacs users can type « and » with C-x 8 < and C-x 8 > respectively)

;; function split and join
(mapcar «list {* 2} {* 3}» '(1 2 3 4)) ; => ((2 3) (4 6) (6 9) (8 12))
(mapcar «and {< 2} #'evenp» '(1 2 3 4)) ; => (NIL NIL NIL T)
(mapcar «+ {* 2} {- _ 1}» '(1 2 3 4)) ; => (2 5 8 11)

Finally, and this is probably too much, even more special brackets can be useful for generating cond/case forms. This can be useful for processing lists of heterogeneous types. For case (not cond) forms, the first element of the ‹›-delimited list is the case "keyform." The other elements are two-element cond/case clauses in which the first is the guard, and the second is the value. Elements of the guard are evaluated at read time to determine if they are functions which should be applied, or literals which should be included verbatim--this could be dangerous in some cases so beware.

;; concise case expressions
(mapcar ‹typecase (number #'1+) (string :str)› '(1 "this" 2 "that"))
                                        ; => (2 :STR 3 :STR)
(mapcar ‹cond (#'evenp {+ 100}) (#'oddp {+ 200})› '(1 2 3 4))
                                        ; => (201 102 203 104)
;; also if/when/unless support
(mapcar ‹if #'evenp #'1+ #'1-› '(1 2 3 4)) ; => (0 3 2 5)

Load CURRY-COMPOSE-READER-MACROS at the REPL with the following

(ql:quickload :curry-compose-reader-macros)
(ql:quickload :named-readtables)
(use-package 'named-readtables)
(in-readtable :curry-compose-reader-macros)

Use CURRY-COMPOSE-READER-MACROS in source by adding NAMED-READTABLES and CURRY-COMPOSE-READER-MACROS to your ASDF file and package and then including the following in source files which use these reader macros.

(in-readtable :curry-compose-reader-macros)

Emacs users may easily treat {}'s, []'s, «»'s, and ‹›'s as parenthesis for paredit commands and SEXP movement with the following configuration.

;; Syntax table
(modify-syntax-entry ?\[ "(]" lisp-mode-syntax-table)
(modify-syntax-entry ?\] ")[" lisp-mode-syntax-table)
(modify-syntax-entry ?\{ "(}" lisp-mode-syntax-table)
(modify-syntax-entry ?\} "){" lisp-mode-syntax-table)
(modify-syntax-entry ?\« "(»" lisp-mode-syntax-table)
(modify-syntax-entry ?\» ")«" lisp-mode-syntax-table)
(modify-syntax-entry ?\‹ "(›" lisp-mode-syntax-table)
(modify-syntax-entry ?\› ")‹" lisp-mode-syntax-table)

;; Paredit keys
(eval-after-load "paredit"
  '(progn
    (define-key paredit-mode-map "[" 'paredit-open-parenthesis)
    (define-key paredit-mode-map "]" 'paredit-close-parenthesis)
    (define-key paredit-mode-map "(" 'paredit-open-bracket)
    (define-key paredit-mode-map ")" 'paredit-close-bracket)
    (define-key paredit-mode-map "«" 'paredit-open-special)
    (define-key paredit-mode-map "»" 'paredit-close-special)
    (define-key paredit-mode-map "‹" 'paredit-open-special)
    (define-key paredit-mode-map "›" 'paredit-close-special)))

Also, Emacs will insert « with C-x 8 < and » with C-x 8 >. Emacs can be instructed to define similar key-bindings for ‹ with the following:

(global-set-key (kbd "\C-x 8 (") (lambda () (interactive) (insert-char #x2039)))
(global-set-key (kbd "\C-x 8 )") (lambda () (interactive) (insert-char #x203A)))

which will cause Emacs to insert ‹ with C-x 8 ( and › with C-x 8 ) respectively.