original.name="UnionType_Valid_2"
======
>>> main.whiley
// Define the 8bit ASCII character
public type char is (int x) where 0 <= x && x <= 255
//
type State is ({char[] input, int pos} s) where s.pos >= 0
type Expr is int | {int[] id}
type SyntaxError is {int[] err}
type SExpr is SyntaxError | Expr

public function isDigit(char c) -> bool:
    return '0' <= c && c <= '9'

function parseTerm(State st) -> (SExpr f1, State f2):
    if st.pos < |st.input|:
        if isDigit(st.input[st.pos]):
            return parseNumber(st)
    return {err: "unknown expression encountered"}, st

function parseNumber(State st) -> (Expr f1, State f2):
    int n = 0
    while st.pos < |st.input| && isDigit(st.input[st.pos]) where st.pos >= 0:
        n = n + (int) (st.input[st.pos] - '0')
        st.pos = st.pos + 1
    return n,st 

public export method test() :
    SExpr f1
    State f2
    (f1,f2) = parseTerm({input: "123", pos: 0})
    assume f1 == 6
    (f1,f2) = parseTerm({input: "abc", pos: 0})
    //
    assume f1 is SyntaxError

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