[m-users.] Confused by action of string.prefix_length

[Sean Charles (emacstheviking)]

I have this code:

:- pred pd_fence(string::in, int::out) is semidet.

pd_fence(S, N) :-
    N = string.prefix_length(is_tilde, S),
    N > 2,
    trace[io(!Dbg)]
    (
        io.format("N is %i for %s\n", [i(N), s(S)], !Dbg)
    ).

And when I run it with some tests strings I get this output, the values seem to be exactly twice the number of tilde characters and I don’t understand why!

./classify_line '# heading1' 'plain text' '```' '```c' '```c<<*>>' '˜˜˜' '˜˜˜{.c}' '˜˜˜˜˜{.c} <<data>>' '˜˜˜˜˜{.c} <<core>>= ../src/filename.c'

plain_text => # heading1
plain_text => plain text
fence_MD(42) => ```
fence_MD(42) => ```c
fence_MD(42) => ```c<<*>>
N is 6 for ˜˜˜
fence_PD(42, 6) => ˜˜˜
N is 6 for ˜˜˜{.c}
fence_PD(42, 6) => ˜˜˜{.c}
N is 10 for ˜˜˜˜˜{.c} <<data>>
fence_PD(42, 10) => ˜˜˜˜˜{.c} <<data>>
N is 10 for ˜˜˜˜˜{.c} <<core>>= ../src/filename.c
fence_PD(42, 10) => ˜˜˜˜˜{.c} <<core>>= ../src/filename.c

Thanks,
Sean.

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