{-# OPTIONS_GHC -fno-warn-type-defaults #-}

{- |
Module     : $Header$
Description: Functions for clac's Read-Evaluate-Print-Loop.
Copyright  : (c) Alexander Berntsen 2015
License    : GPL-3

Maintainer : alexander@plaimi.net
-} module Clac.REPL where

import Safe
  (
  readMay,
  )

import Plailude

inlet :: Eq a => a -> b -> [(a, b)] -> [(a, b)]
-- | Insert a let into the given stack of lets. If the variable is already
-- bound, update it by 'unlet'ing it first, and then let it.
--
-- > #let a = 1 -- stack is [("a","1")], i.e. a bound to 1
-- > #let a = 2 -- stack is [("a","2")]
inlet k n ls = case lookup k ls of
                Just _  -> (k, n) : unlet k ls
                Nothing -> (k, n) : ls

unlet :: Eq a => a -> [(a, b)] -> [(a, b)]
-- | Remove a let binding from the given stack of lets. Noop if there's
-- nothing to unlet.
--
-- > #let a = 1 -- stack is [("a", "1")], i.e. a bound to 1
-- > #let b = 2 -- stack is [("b","2"),("a", "1")]
-- > #unlet a   -- stack is [("b","2")]
unlet k = go []
  where go u []                       = u
        go u (l@(m,_):ls) | m == k    = u ++ ls
                          | otherwise = go (u ++ [l]) ls

repLets :: Eq a => [(a, a)] -> [a] -> [a]
-- | Matches the passed in let bindings with the passed in equation, and
-- reduces any found bindings with their value.
--
-- > #let a = 1
-- > a a + -- this is turned into [["1","1","+"]]
repLets lets = go []
  where go as [] = as
        go as (l:bs) = case lookup l lets of
                         Just m  -> go (as ++ [m]) bs
                         Nothing -> go (as ++ [l]) bs

get :: Show a => [a] -> String -> String
-- | Used the passed in 'String' as an Integer index into the passed in stack
-- of answers. Breaks the equation if the string can't be parsed.
--
-- > 0     -- stack is now [0.0]
-- > 0 ans -- gets 0.0 from the stack
-- > 3 ans -- stack is now [0.0,0.0], no 3rd element -- broken equation
get as i = maybe "bork" show $ ((as !?) . floor) =<< readMay i

repAns :: Show a => [a] -> [String] -> [String]
-- | Matches the passed in ans stack with the passed in equation, and
-- reduces any found applications of the ans function with the value
--
-- > 1     -- ans stack is now [1.0]
-- > 0 ans -- this is turned into [["1"]]
repAns ans = go []
  where go as []                    = as
        go as (n:m:bs) | m == "ans" = go [] (as ++ (ans `get` n) : bs)
                       | otherwise  = go (as ++ [n] ++ [m]) bs
        go as [a]                   = as ++ [a]