-- pascal :: Generator [[Int]]
on pascal()
    script nextRow
        on |λ|(row)
            zipWith(my plus, {0} & row, row & {0})
        end |λ|
    end script
    iterate(nextRow, {1})
end pascal


on run
    showPascal(take(7, pascal()))
end run


-- showPascal :: [[Int]] -> String
on showPascal(xs)
    set w to length of intercalate("   ", item -1 of xs)
    script align
        on |λ|(x)
            |center|(w, space, intercalate("   ", x))
        end |λ|
    end script
    unlines(map(align, xs))
end showPascal


-- GENERIC ABSTRACTIONS -------------------------------------------------------

-- center :: Int -> Char -> String -> String
on |center|(n, cFiller, strText)
    set lngFill to n - (length of strText)
    if lngFill > 0 then
        set strPad to replicate(lngFill div 2, cFiller) as text
        set strCenter to strPad & strText & strPad
        if lngFill mod 2 > 0 then
            cFiller & strCenter
        else
            strCenter
        end if
    else
        strText
    end if
end |center|

-- intercalate :: String -> [String] -> String
on intercalate(sep, xs)
    set {dlm, my text item delimiters} to {my text item delimiters, sep}
    set s to xs as text
    set my text item delimiters to dlm
    return s
end intercalate

-- iterate :: (a -> a) -> a -> Generator [a]
on iterate(f, x)
    script
        property v : missing value
        property g : mReturn(f)'s |λ|
        on |λ|()
            if missing value is v then
                set v to x
            else
                set v to g(v)
            end if
            return v
        end |λ|
    end script
end iterate

-- length :: [a] -> Int
on |length|(xs)
    set c to class of xs
    if list is c or string is c then
        length of xs
    else
        2 ^ 30 -- (simple proxy for non-finite)
    end if
end |length|

-- map :: (a -> b) -> [a] -> [b]
on map(f, xs)
    tell mReturn(f)
        set lng to length of xs
        set lst to {}
        repeat with i from 1 to lng
            set end of lst to |λ|(item i of xs, i, xs)
        end repeat
        return lst
    end tell
end map

-- min :: Ord a => a -> a -> a
on min(x, y)
    if y < x then
        y
    else
        x
    end if
end min

-- Lift 2nd class handler function into 1st class script wrapper
-- mReturn :: First-class m => (a -> b) -> m (a -> b)
on mReturn(f)
    if class of f is script then
        f
    else
        script
            property |λ| : f
        end script
    end if
end mReturn

-- plus :: Num -> Num -> Num
on plus(a, b)
    a + b
end plus

-- Egyptian multiplication - progressively doubling a list, appending
-- stages of doubling to an accumulator where needed for binary
-- assembly of a target length
-- replicate :: Int -> a -> [a]
on replicate(n, a)
    set out to {}
    if n < 1 then return out
    set dbl to {a}

    repeat while (n > 1)
        if (n mod 2) > 0 then set out to out & dbl
        set n to (n div 2)
        set dbl to (dbl & dbl)
    end repeat
    return out & dbl
end replicate

-- take :: Int -> [a] -> [a]
-- take :: Int -> String -> String
on take(n, xs)
    set c to class of xs
    if list is c then
        if 0 < n then
            items 1 thru min(n, length of xs) of xs
        else
            {}
        end if
    else if string is c then
        if 0 < n then
            text 1 thru min(n, length of xs) of xs
        else
            ""
        end if
    else if script is c then
        set ys to {}
        repeat with i from 1 to n
            set end of ys to xs's |λ|()
        end repeat
        return ys
    else
        missing value
    end if
end take

-- unlines :: [String] -> String
on unlines(xs)
    set {dlm, my text item delimiters} to ¬
        {my text item delimiters, linefeed}
    set str to xs as text
    set my text item delimiters to dlm
    str
end unlines

-- unwords :: [String] -> String
on unwords(xs)
    set {dlm, my text item delimiters} to {my text item delimiters, space}
    set s to xs as text
    set my text item delimiters to dlm
    return s
end unwords

-- zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
on zipWith(f, xs, ys)
    set lng to min(|length|(xs), |length|(ys))
    if 1 > lng then return {}
    set xs_ to take(lng, xs) -- Allow for non-finite
    set ys_ to take(lng, ys) -- generators like cycle etc
    set lst to {}
    tell mReturn(f)
        repeat with i from 1 to lng
            set end of lst to |λ|(item i of xs_, item i of ys_)
        end repeat
        return lst
    end tell
end zipWith