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;;; Nujel - Copyright (C) 2020-2021 - Benjamin Vincent Schulenburg
;;; This project uses the MIT license, a copy should be included under /LICENSE
;;;
;;; Contains an optimization pass that does some simple constant folding
;;;
;;; To accomplish this we walk an expression tree, searching for pure functions
;;; with constant arguments, since these should be safe to replace with their
;;; respective results.
;;;
;;; Pure functions are those whose :pure meta value is #t, meaning we trust
;;; the developer to determine which functions are safe to constant fold, in
;;; future versions we might be able to infer if a given fn is pure.
[defn constant-fold/constant? [expr]
[and [not [pair? expr]]
[not [symbol? expr]]
#t]]
[defn constant-fold/pure? [fun]
[meta fun :pure]]
[defn constant-fold/resolve [sym]
[when-not [symbol? sym] [return sym]]
[resolve sym]]
[defn constant-fold/args [expr]
[when-not expr [return expr]]
[if [pair? [car expr]]
[cons [constant-fold [car expr]]
[constant-fold/args [cdr expr]]]
[cons [car expr]
[constant-fold/args [cdr expr]]]]]
[defn constant-fold [expr]
"Will try and evaluate as many constant as possible to make the expression simpler."
[when-not [pair? expr] [return expr]]
[def folded-fun [car expr]]
[when [= 'quote folded-fun] [return expr]]
[def folded-args [constant-fold/args [cdr expr]]]
[if [and [constant-fold/pure? folded-fun]
[every? folded-args constant-fold/constant?]]
[try [fn [] [cons folded-fun folded-args]]
[apply [constant-fold/resolve folded-fun] folded-args]]
[cons folded-fun folded-args]]]
