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+--- Lua operators available as functions.
+--
+-- (similar to the Python module of the same name)
+--
+-- There is a module field `optable` which maps the operator strings
+-- onto these functions, e.g. `operator.optable['()']==operator.call`
+--
+-- Operator strings like '>' and '{}' can be passed to most Penlight functions
+-- expecting a function argument.
+--
+-- @module pl.operator
+
+local strfind = string.find
+
+local operator = {}
+
+--- apply function to some arguments **()**
+-- @param fn a function or callable object
+-- @param ... arguments
+function operator.call(fn,...)
+ return fn(...)
+end
+
+--- get the indexed value from a table **[]**
+-- @param t a table or any indexable object
+-- @param k the key
+function operator.index(t,k)
+ return t[k]
+end
+
+--- returns true if arguments are equal **==**
+-- @param a value
+-- @param b value
+function operator.eq(a,b)
+ return a==b
+end
+
+--- returns true if arguments are not equal **~=**
+ -- @param a value
+-- @param b value
+function operator.neq(a,b)
+ return a~=b
+end
+
+--- returns true if a is less than b **<**
+-- @param a value
+-- @param b value
+function operator.lt(a,b)
+ return a < b
+end
+
+--- returns true if a is less or equal to b **<=**
+-- @param a value
+-- @param b value
+function operator.le(a,b)
+ return a <= b
+end
+
+--- returns true if a is greater than b **>**
+-- @param a value
+-- @param b value
+function operator.gt(a,b)
+ return a > b
+end
+
+--- returns true if a is greater or equal to b **>=**
+-- @param a value
+-- @param b value
+function operator.ge(a,b)
+ return a >= b
+end
+
+--- returns length of string or table **#**
+-- @param a a string or a table
+function operator.len(a)
+ return #a
+end
+
+--- add two values **+**
+-- @param a value
+-- @param b value
+function operator.add(a,b)
+ return a+b
+end
+
+--- subtract b from a **-**
+-- @param a value
+-- @param b value
+function operator.sub(a,b)
+ return a-b
+end
+
+--- multiply two values __*__
+-- @param a value
+-- @param b value
+function operator.mul(a,b)
+ return a*b
+end
+
+--- divide first value by second **/**
+-- @param a value
+-- @param b value
+function operator.div(a,b)
+ return a/b
+end
+
+--- raise first to the power of second **^**
+-- @param a value
+-- @param b value
+function operator.pow(a,b)
+ return a^b
+end
+
+--- modulo; remainder of a divided by b **%**
+-- @param a value
+-- @param b value
+function operator.mod(a,b)
+ return a%b
+end
+
+--- concatenate two values (either strings or `__concat` defined) **..**
+-- @param a value
+-- @param b value
+function operator.concat(a,b)
+ return a..b
+end
+
+--- return the negative of a value **-**
+-- @param a value
+function operator.unm(a)
+ return -a
+end
+
+--- false if value evaluates as true **not**
+-- @param a value
+function operator.lnot(a)
+ return not a
+end
+
+--- true if both values evaluate as true **and**
+-- @param a value
+-- @param b value
+function operator.land(a,b)
+ return a and b
+end
+
+--- true if either value evaluate as true **or**
+-- @param a value
+-- @param b value
+function operator.lor(a,b)
+ return a or b
+end
+
+--- make a table from the arguments **{}**
+-- @param ... non-nil arguments
+-- @return a table
+function operator.table (...)
+ return {...}
+end
+
+--- match two strings **~**.
+-- uses @{string.find}
+function operator.match (a,b)
+ return strfind(a,b)~=nil
+end
+
+--- the null operation.
+-- @param ... arguments
+-- @return the arguments
+function operator.nop (...)
+ return ...
+end
+
+---- Map from operator symbol to function.
+-- Most of these map directly from operators;
+-- But note these extras
+--
+-- * __'()'__ `call`
+-- * __'[]'__ `index`
+-- * __'{}'__ `table`
+-- * __'~'__ `match`
+--
+-- @table optable
+-- @field operator
+ operator.optable = {
+ ['+']=operator.add,
+ ['-']=operator.sub,
+ ['*']=operator.mul,
+ ['/']=operator.div,
+ ['%']=operator.mod,
+ ['^']=operator.pow,
+ ['..']=operator.concat,
+ ['()']=operator.call,
+ ['[]']=operator.index,
+ ['<']=operator.lt,
+ ['<=']=operator.le,
+ ['>']=operator.gt,
+ ['>=']=operator.ge,
+ ['==']=operator.eq,
+ ['~=']=operator.neq,
+ ['#']=operator.len,
+ ['and']=operator.land,
+ ['or']=operator.lor,
+ ['{}']=operator.table,
+ ['~']=operator.match,
+ ['']=operator.nop,
+}
+
+return operator