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https://github.com/janet-lang/janet
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Tail recursive match implementation.
This implementation uses multiple passes on patterns to remove the need for a sentinel value to check if there was a match. This also re-uses extracted subpatterns for complicated patterns.
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Calvin Rose
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@ -2,6 +2,7 @@
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All notable changes to this project will be documented in this file.
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## Unreleased - ???
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- `match` macro implemtentation cahned to be tail recursive.
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- Adds a :preload loader which allows one to manually put things into `module/cache`.
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- Add `buffer/push` function.
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- Backtick delimited strings and buffers are now reindented based on the column of the
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@ -1559,85 +1559,9 @@
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###
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###
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(defmacro- with-idemp
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`Return janet code body that has been prepended
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with a binding of form to atom. If form is a non-idempotent
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form (a function call, etc.), make sure the resulting
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code will only evaluate once, even if body contains multiple
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copies of binding. In body, use binding instead of form.`
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[binding form & body]
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(def $result (gensym))
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(def $form (gensym))
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~(do
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(def ,$form ,form)
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(def ,binding (if (idempotent? ,$form) ,$form (gensym)))
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(def ,$result (do ,;body))
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(if (= ,$form ,binding)
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,$result
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(tuple 'do (tuple 'def ,binding ,$form) ,$result))))
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# Sentinel value for mismatches
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(def- sentinel ~',(gensym))
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(defn- match-1
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[pattern expr onmatch seen]
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(cond
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(= '_ pattern)
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(onmatch)
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(symbol? pattern)
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(if (in seen pattern)
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~(if (= ,pattern ,expr) ,(onmatch) ,sentinel)
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(do
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(put seen pattern true)
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~(do (def ,pattern ,expr) ,(onmatch))))
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(and (tuple? pattern) (= :parens (tuple/type pattern)))
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(if (= (get pattern 0) '@)
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# Unification with external values
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~(if (= ,(get pattern 1) ,expr) ,(onmatch) ,sentinel)
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(match-1
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(in pattern 0) expr
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(fn []
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~(if (and ,;(tuple/slice pattern 1)) ,(onmatch) ,sentinel)) seen))
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(indexed? pattern)
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(do
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(def len (length pattern))
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(var i -1)
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(with-idemp
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$arr expr
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~(if (,indexed? ,$arr)
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(if (< (,length ,$arr) ,len)
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,sentinel
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,((fn aux []
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(++ i)
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(if (= i len)
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(onmatch)
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(match-1 (in pattern i) (tuple in $arr i) aux seen)))))
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,sentinel)))
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(dictionary? pattern)
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(do
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(var key nil)
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(with-idemp
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$dict expr
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~(if (,dictionary? ,$dict)
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,((fn aux []
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(set key (next pattern key))
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(def $val (gensym))
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(if (= key nil)
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(onmatch)
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~(do (def ,$val (,get ,$dict ,key))
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,(match-1 [(in pattern key) [not= nil $val]] $val aux seen)))))
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,sentinel)))
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:else ~(if (= ,pattern ,expr) ,(onmatch) ,sentinel)))
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(defmacro match
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`Pattern matching. Match an expression x against
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```
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Pattern matching. Match an expression x against
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any number of cases. Each case is a pattern to match against, followed
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by an expression to evaluate to if that case is matched. A pattern that is
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a symbol will match anything, binding x's value to that symbol. An array
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@ -1646,22 +1570,152 @@
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values in x. A tuple pattern will match if it's first element matches, and the following
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elements are treated as predicates and are true. The last special case is
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the '_ symbol, which is a wildcard that will match any value without creating a binding.
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Any other value pattern will only match if it is equal to x.`
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Any other value pattern will only match if it is equal to x.
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```
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[x & cases]
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(with-idemp $x x
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(def len (length cases))
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(def len-1 (dec len))
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((fn aux [i]
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(cond
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(= i len-1) (in cases i)
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(< i len-1) (with-syms [$res]
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~(if (= ,sentinel (def ,$res ,(match-1 (in cases i) $x (fn [] (in cases (inc i))) @{})))
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,(aux (+ 2 i))
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,$res)))) 0)))
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(undef sentinel)
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(undef match-1)
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(undef with-idemp)
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# Partition body into sections.
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(def oddlen (odd? (length cases)))
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(def else (if oddlen (last cases)))
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(def patterns (partition 2 (if oddlen (slice cases 0 -2) cases)))
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# Keep an array for accumulating the compilation output
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(def x-sym (if (idempotent? x) x (gensym)))
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(def accum @[])
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(if (not= x x-sym) (array/push accum ['def x-sym x]))
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# Table of gensyms
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(def symbols @{[nil nil] x-sym})
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(def length-symbols @{})
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(defn emit [x] (array/push accum x))
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(defn emit-branch [condition result] (array/push accum :branch condition result))
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(defn get-sym
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[parent-sym key]
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(def symbol-key [parent-sym key])
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(or (get symbols symbol-key)
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(let [s (gensym)]
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(put symbols symbol-key s)
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(emit ['def s [get parent-sym key]])
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s)))
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(defn get-length-sym
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[parent-sym]
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(or (get length-symbols parent-sym)
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(let [s (gensym)]
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(put length-symbols parent-sym s)
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(emit ['def s ['if [indexed? parent-sym] [length parent-sym]]])
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s)))
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(defn visit-pattern-1
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[b2g parent-sym key pattern]
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(if (= pattern '_) (break))
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(def s (get-sym parent-sym key))
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(def t (type pattern))
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(def isarr (or (= t :array) (and (= t :tuple) (= (tuple/type pattern) :brackets))))
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(cond
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# match local binding
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(= t :symbol)
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(if-let [x (in b2g pattern)]
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(array/push x s)
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(put b2g pattern @[s]))
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# match data structure template
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(or isarr (= t :struct) (= t :table))
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(do
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(when isarr (get-length-sym s))
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(eachp [i sub-pattern] pattern
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(visit-pattern-1 b2g s i sub-pattern)))
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# match global unification
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(and (= t :tuple) (= 2 (length pattern)) (= '@ (pattern 0)))
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(break)
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# match predicated binding
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(and (= t :tuple) (>= (length pattern) 2))
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(do
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(visit-pattern-1 b2g parent-sym key (pattern 0)))))
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(defn visit-pattern-2
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[anda gun preds parent-sym key pattern]
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(if (= pattern '_) (break))
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(def s (get-sym parent-sym key))
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(def t (type pattern))
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(def isarr (or (= t :array) (and (= t :tuple) (= (tuple/type pattern) :brackets))))
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(when isarr
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(array/push anda (get-length-sym s))
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(array/push anda [<= (length pattern) (get-length-sym s)]))
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(cond
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# match data structure template
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(or isarr (= t :struct) (= t :table))
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(eachp [i sub-pattern] pattern
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(when (not= t :array)
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(array/push anda [not= nil (get-sym s i)]))
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(visit-pattern-2 anda gun preds s i sub-pattern))
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# match local binding
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(= t :symbol) (break)
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# match global unification
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(and (= t :tuple) (= 2 (length pattern)) (= '@ (pattern 0)))
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(if-let [x (in gun (pattern 1))]
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(array/push x s)
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(put gun (pattern 1) @[s]))
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# match predicated binding
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(and (= t :tuple) (>= (length pattern) 2))
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(do
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(array/push preds ;(slice pattern 1))
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(visit-pattern-2 anda gun preds parent-sym key (pattern 0)))
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# match literal
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(array/push anda ['= s pattern])))
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# Compile the patterns
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(each [pattern expression] patterns
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(def b2g @{})
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(def gun @{})
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(def preds @[])
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(visit-pattern-1 b2g nil nil pattern)
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(def anda @['and])
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(visit-pattern-2 anda gun preds nil nil pattern)
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# Local unification
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(def unify @[])
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(each syms b2g
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(when (< 1 (length syms))
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(array/push unify [= ;syms])))
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# Global unification
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(eachp [binding syms] gun
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(array/push unify [= binding ;syms]))
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(sort unify)
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(array/concat anda unify)
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# Final binding
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(def defs (seq [[k v] :in (sort (pairs b2g))] ['def k (first v)]))
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# Predicates
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(unless (empty? preds)
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(def pred-join ~(do ,;defs (and ,;preds)))
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(array/push anda pred-join))
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(emit-branch (tuple/slice anda) ['do ;defs expression]))
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# Expand branches
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(def stack @[else])
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(each el (reverse accum)
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(if (= :branch el)
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(let [condition (array/pop stack)
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truthy (array/pop stack)
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if-form ~(if ,condition ,truthy
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,(case (length stack)
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0 nil
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1 (stack 0)
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~(do ,;(reverse stack))))]
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(array/remove stack 0 (length stack))
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(array/push stack if-form))
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(array/push stack el)))
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~(do ,;(reverse stack)))
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###
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###
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