A iteration library for Lua
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Lua Iteration Library

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luaiter is a rewritten version of luafun: a high-performance functional programming library for Lua designed with LuaJIT's trace compiler in mind. luaiter focus plain Lua performance improve and follows the standard Lua iteration protocol.

luaiter has the same License as Lua itself.

Some improves:

  • avoid any memory allocation when iteration.
  • use standard iteration protocol.
  • support Lua 5.3 bit operators.
  • add more useful functions like scan and flatmap.
  • add a powerful selector interface for quick-and-dirty lambda function support.

The standard iteration protocol

luafun library use a custom protocol for iteration, makes using other Lua-spec iterator e.g. io.lines(), string.gmatch difficult, it requires a iteration state variable. luaiter follows the standard protocol without the per-iteration state variable:

for var1, ... in iter, state, init do

The first return value of iter function var1 used as the state variable, but it's meaningful: If iter function is stateful, i.e. each iteration will change the state content, then var1 may occurs the duplicate value in iterations. In this case, init will be nil to indicate the beginning of the iteration (note that nil will never occurs in iteration: it means the end of stream). Otherwise, the iter will be stateless, means var1 will never repetition during iteration.

  • The stateful iterator example: map (remember the original iterator var1).
  • The stateless iterator example: range (it only use previous var1 to detect the next var1).

The selector interface

luaiter has a very special selector interface, the underscore iter._. This is a special object that has several functinal:

  • _[1] ... _[9] called selector, they can be used as function that select it's 1st...9th argument, e.g. _[5] same as function(a, b, c, d, e) return e end. They could shorten as iter._1 to iter._9

  • _1 to _9 could used in expression, in this case the expression will return a function that do the calculation, and _1 ... _9 means the order of arguments, e.g. _3 + _1 * _2 same as function(a, b, c) return c + a * b end. This will support all Lua operator that could override by metatable, including _1[_2].

  • if use _ as a function, it could return a function that call the _'s single argument, e.g. _(print)(_2, _1) same as function(a, b) return print(b, a) end, all underscore expression could be used in all place in call, e.g. _(_2.each)(_3, _1*_4) same as function(a, b, c ,d) return b.each(c, a*d) end

  • _.self returns a table-object, use _.self(obj).each(_1, _2) same as function(a, b) return obj:each(a, b) end.

  • _.dots same as ..., if use _.dots in a expression/call, the generated function will accept vararg arguments.

  • _.land, _.lor, _.lnot, _.andor same as and, or, not operator and a and b or c expression.

  • used of _ and _1 to _9 may cause load/loadstring when first call the generated function, every time the underscore expression calculated, a new function will load/loadstring from expression, so don't write expression in loop. Generate the function, and store it in the iterator will cache the generated function.

A example:

> -- Functional style
>  print(reduce(_1+_2, 0, map(_1^2, range(100))))

> -- Object-oriented style
> print(range(100):map(_1^2):reduce(_1+_2))

The interface convention

All functions that accept a iterator may used as the method of iterator object. Iterator usually place at the end of interface, when used as methods, the last iterator will be self, e.g. map function has signature: map(func, iter), So use map as a method can call like this: iter:map(func)

If a function accept multiple iterators, the first will be the self iterator, e.g. zip(iter, iters...) maybe called as iter:zip(iters...)

If a function doesn't accept a iterator, it can not used as the method of iterator object.

The iterators


  • range([[first,] last[, step]])
  • rand([first, last])
  • str(string)
  • array(table)
  • resolve(...)
  • dup(...)
  • zeros() == dup(0)
  • ones() == dup(1)


  • take(n, iter)
  • drop(n, iter)
  • slice(first, last, iter)


  • map(func, iter)
  • flatmap(func, iter)
  • scan(func, init, iter)
  • group(n, iter)
  • groupby(func, iter)


  • zip(iters...)
  • interleave(iters...)
  • chain(iters...)
  • cycle(iter)


  • takewhile(func, iter)
  • dropwhile(func, iter)
  • filter(func, iter)
  • fitlerout(func, iter)


  • each(func, iter)
  • reduce(func, iter)
  • index(func, iter)
  • collect(t, iter)
  • concat(delim, iter)
  • count(iter)
  • isempty(iter)
  • all(func, iter)
  • any(func, iter)