Higher-order functions in Lua

map and filter are three-line helpers over ipairs.

-- Higher-order helpers are three lines each in Lua.
local function map(t, f)
  local out = {}
  for i, v in ipairs(t) do out[i] = f(v) end
  return out
end

local function filter(t, keep)
  local out = {}
  for _, v in ipairs(t) do
    if keep(v) then out[#out + 1] = v end
  end
  return out
end

local hauls = { 12, 45, 3, 78, 22 }
local doubled = map(hauls, function(g) return g * 2 end)
print(table.concat(doubled, " "))
local rich = filter(hauls, function(g) return g > 20 end)
print("Big hauls: " .. table.concat(rich, ", "))

How it works

1. map builds a new table by applying f slot by slot.
2. filter appends with out[#out + 1] when the test passes.
3. Anonymous functions inline the transforms at the call.

Keywords and builtins used here

• do
• end
• f
• filter
• for
• function
• if
• in
• ipairs
• keep
• local
• map
• print
• return
• then

The run, in numbers

Lines

20

Characters to type

514

Tokens

149

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 138 seconds.

Type this snippet

Step 2 of 3 in Closures, step 15 of 27 in Language basics.

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