module VariableExpansion where

-- SPDX-FileCopyrightText: Chris Pressey, the original author of this work, has dedicated it to the public domain.
-- For more information, please refer to <https://unlicense.org/>
-- SPDX-License-Identifier: Unlicense

expand "" alist =
    ""
expand ('$':'{':rest) alist =
    let
        (name, rest') = getName rest
    in
        case lookup name alist of
            Just value ->
                value ++ (expand rest' alist)
            Nothing ->
                expand rest' alist
    where
        getName "" =
            ("", "")
        getName ('}':rest) =
            ("", rest)
        getName (c:rest) =
            let
                (remainder, rest') = getName rest
            in
                ((c:remainder), rest')
expand (c:rest) alist =
    (c:expand rest alist)

test =
    [
      expand "Hello, ${name}!" [("name", "Joe")],
      expand "${a}, ${b}" [("b", "A"), ("a", "B")],
      expand "${no}${way}" [],
      expand "${hmm" [("hmm", "Oh!")]
    ]