#1543: flag to define transpose [] == _|_
------------------------------+---------------------------------------------
Reporter: guest | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.6.1
Severity: minor | Keywords:
Difficulty: Unknown | Os: Unknown
Testcase: | Architecture: Unknown
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I use transpose a lot as an "unzip" function for lists. For example,
suppose:
{{{
foo :: [[String]]
}}}
is a list of rows (say, of lenght 2) obtained from a SQL query. To
transform it into a list of 2 columns, I simply write transpose foo. But,
if foo is empty, I obtain... [[],[]]? No, I obtain []. I want the program
to raise an error and stop, instead of (perhaps) produce a pattern match
failure later, because transpose [] can arguably be [[]] or [[],[]] or
[[],[],[]] or ... at least in some situations where it is used to "unzip"
lists of lists.
I could use foo :: [(String, String)] or foo :: [(String, String, String)]
etc., but [ [String]] is simpler and more general, and I can use map, etc.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1543>
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