stand-in-language/simpleplus.tel at master · Stand-In-Language/stand-in-language · GitHub

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import Prelude

validDigit = \x -> and ($47 left x) (not ($57 left x))

validInput = \x -> assert (and (validDigit (left x))
                               (and (validDigit (left (right (right x))))
                                    (not ($3 right x))))
                          "invalid input"

toI = \x -> $48 left x

doAdd = \x y -> c2d (plus (d2c (toI x)) (d2c (toI y)))

main = \input -> let inString : validInput = left input
                     x = left inString
                     y = left (right (right inString))
                     r = doAdd x y
                     dr = if dMinus r 9
                          then concat ["1", $49 succ (dMinus r 10)]
                          else $49 succ r
                 in if not input
                    then ("enter two digits separated by a space", 1)
                    else (concat [succ x, " plus ", succ y, " is ", dr], 0)