Monads/Maybe monad: Difference between revisions
Added Lua code
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=={{header|Lua}}==
{{works with|lua|5.3}}
<lang lua>
-- None is represented by an empty table. Some is represented by any
-- array with one element. You SHOULD NOT compare maybe values with the
-- Lua operator == because it will give incorrect results. Use the
-- functions isMaybe(), isNone(), and isSome().
-- define once for efficiency, to avoid many different empty tables
local NONE = {}
local function unit(x) return { x } end
local Some = unit
local function isNone(mb) return #mb == 0 end
local function isSome(mb) return #mb == 1 end
local function isMaybe(mb) return isNone(mb) or isSome(mb) end
-- inverse of Some(), extract the value from the maybe; get(Some(x)) === x
local function get(mb) return mb[1] end
function maybeToStr(mb)
return isNone(mb) and "None" or ("Some " .. tostring(get(mb)))
end
local function bind(mb, ...) -- monadic bind for multiple functions
local acc = mb
for _, fun in ipairs({...}) do -- fun should be a monadic function
assert(type(fun) == "function")
if isNone(acc) then return NONE
else acc = fun(get(acc)) end
end
return acc
end
local function fmap(mb, ...) -- monadic fmap for multiple functions
local acc = mb
for _, fun in ipairs({...}) do -- fun should be a regular function
assert(type(fun) == "function")
if isNone(acc) then return NONE
else acc = Some(fun(get(acc))) end
end
return acc
end
-- ^^^ End of generic maybe monad functionality ^^^
--- vvv Start of example code vvv
local function time2(x) return x * 2 end
local function plus1(x) return x + 1 end
local answer
answer = fmap(Some(3), time2, plus1, time2)
assert(get(answer)==14)
answer = fmap(NONE, time2, plus1, time2)
assert(isNone(answer))
local function safeReciprocal(x)
if x ~= 0 then return Some(1/x) else return NONE end
end
local function safeRoot(x)
if x >= 0 then return Some(math.sqrt(x)) else return NONE end
end
local function safeLog(x)
if x > 0 then return Some(math.log(x)) else return NONE end
end
local function safeComputation(x)
return bind(safeReciprocal(x), safeRoot, safeLog)
end
local function map(func, table)
local result = {}
for key, val in pairs(table) do
result[key] = func(val)
end
return result
end
local inList = {-2, -1, -0.5, 0, math.exp (-1), 1, 2, math.exp (1), 3, 4, 5}
print("input:", table.concat(map(tostring, inList), ", "), "\n")
local outList = map(safeComputation, inList)
print("output:", table.concat(map(maybeToStr, outList), ", "), "\n")
</lang>
=={{header|Perl 6}}==
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