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From "Brian Candler (JIRA)" <>
Subject [jira] Commented: (COUCHDB-336) Would like errors in map function to be reported to client
Date Tue, 05 May 2009 21:23:36 GMT


Brian Candler commented on COUCHDB-336:

As an additional point: the error messages currently returned for a badly-formed map or reduce
function are not as good as the standalone js is capable of producing. For example:

$ curl -X POST -d '{"map":"function(doc){val val=doc.k; emit(val||null,null);}"}'
{"error":"compilation_error","reason":"expression does not eval to a function. (function(doc){val
val=doc.k; emit(val||null,null);})"}

It's very common for a syntax error to return only "expression does not eval to a function".
However, pasting the same code into js gives a much more informative error, indicating both
the nature of the error and its location.

js> function(doc){val val=doc.k; emit(val||null,null);}
typein:1: SyntaxError: missing ; before statement:
typein:1: function(doc){val val=doc.k; emit(val||null,null);}
typein:1: ..................^

> Would like errors in map function to be reported to client
> ----------------------------------------------------------
>                 Key: COUCHDB-336
>                 URL:
>             Project: CouchDB
>          Issue Type: Wish
>         Environment: svn trunk 0.10.0a768591
>            Reporter: Brian Candler
>            Priority: Minor
> At the moment, if a runtime error occurs in a map function, the document is silently
excluded from the view and the client is none the wiser.
> $ curl -X PUT
> {"ok":true}
> $ curl -d '{"foo":"bar"}' -X PUT
> {"ok":true,"id":"doc1","rev":"1-3413565576"}
> $ curl -d '{"food":"bar"}' -X PUT
> {"ok":true,"id":"doc2","rev":"1-2088327893"}
> # Example 1: map function calls undefined function
> $ curl -d '{"map":"function(doc) { emitt(null,null); }"}' -H 'Content-Type: application/json'
> {"total_rows":0,"rows":[]}
> # Example 2: map function attempts to emit 'undefined'
> $ curl -d '{"map":"function(doc) { emit(doc._id,; }"}' -H 'Content-Type: application/json'
> {"total_rows":1,"offset":0,"rows":[
> {"id":"doc1","key":"doc1","value":"bar"}
> ]}
> Of course, errors are available in the server log:
> [Tue, 28 Apr 2009 08:40:39 GMT] [info] [<0.1276.0>] OS Process Log Message: function
raised exception (ReferenceError: emitt is not defined) with doc._id doc1
> [Tue, 28 Apr 2009 08:43:44 GMT] [info] [<0.1276.0>] OS Process Log Message: function
raised exception (Cannot encode 'undefined' value as JSON) with doc._id doc2
> But the client cannot see these, and worse, gets what appears to be a valid result set.
> I think there are two cases to consider.
> - in temporary views, it ought to be easy to do something: e.g. (1) return the full set
of failed docs with their errors, or (2) return a count of failed docs, or (3) just abort
on the first error
> - in permanent views, it might also be useful to return this information, but it could
be expensive to persist. Also, when you're querying for key X you might not want to see a
whole load of errors for unrelated docs. An error count might be OK though, similar to total_rows.
> The current behaviour may be useful, although I personally wouldn't want to rely on it,
especially as it generates errors in the log. That is, I would rather be forced to rewrite
example 2 as
>   function(doc) { if ( { emit(doc._id,; } }
> or
>   function(doc) { emit(doc._id, || null); }
> as appropriate.
> Workaround: users can manually wrap each of their map functions in a catcher which emits
a sentinel key/value pair containing the error. However they would still have to query the
key range which includes the error messages.

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