问题描述
我想将DATA fom表mnesia导出到txt文件
I want to export DATA fom table mnesia to the txt file
我尝试使用以下代码：
exporttxt()->
F = fun(T) -> mensia:foldl(fun(X,Acc) -> [X|Acc] end, [],T) end,
{atomic,L} = mnesia:transaction(F(user)),
file:write_file("test.txt",[io_lib:format("~p\t~p\t~p~n",[F1,F2,F3]) ||
#user{id = F1,adress = F2,birthday = F3} <- L]).
但是当我测试这个函数时，我有这个错误：
but when I test this function I have this error :
Erlang R13B03 (erts-5.7.4) [source] [rq:1] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.7.4 (abort with ^G)
1> model:exporttxt().
** exception error: undefined function mensia:foldl/3
in function model:exporttxt/0
2>
如您所见，我使用erlang版本13
as you see I work with erlang version 13
我现在尝试使用以下代码：
exporttxt()->
F = fun(T) -> mnesia:foldl(fun(X,Acc) -> [X|Acc] end, [],T) end,
{atomic,L} = mnesia:transaction(F(user)),
file:write_file("test.txt",[io_lib:format("~p\t~p\t~p~n",[F1,F2,F3]) ||
#user{id = F1,adress = F2,birthday = F3} <- L]).
但我有这个错误：
** exception exit: {aborted,no_transaction}
in function mnesia:abort/1
in call from model:exporttxt/0
我也尝试：
exporttxt()->
F = fun(T) -> mnesia:foldl(fun(X,Acc) -> [X|Acc] end, [],T) end,
{atomic,L} = mnesia:transaction(F),
file:write_file("test.txt",[io_lib:format("~p\t~p\t~p~n",[F1,F2,F3]) ||
#user{id = F1,adress = F2,birthday = F3} <- L]).
但我有这个错误：
** exception error: no match of right hand side value
{aborted,{{badarity,{#Fun<model.208.16694406>,[]}},
[{mnesia_tm,apply_fun,3},
{mnesia_tm,execute_transaction,5},
{model,exporttxt,0},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_exprs,6},
{shell,eval_loop,3}]}}
in function model:exporttxt/0
推荐答案
mnesia:transaction(F(user))
这是错误的，花一点时间看看。 mnesia：transaction / 1期望一个函数，你没有传递一个函数，你通过F（user）的结果是..（记住erlang使用严格的评估，所以它评估F用户） mnesia：transaction / 1）
that is wrong, take a minute and look at it. mnesia:transaction/1 expect a function, you aren't passing it a function, you pass whatever the result of F(user) is.. (remember erlang uses strict evaluation, so it evaluates F(user) before mnesia:transaction/1).
在你的情况下，对F（用户）的调用失败，因为它预计会在里面运行一个交易。
In your case that call to F(user) fails because it was expected to run inside a transaction.
这篇关于在erlang中调用未定义的函数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！