The reason cassava is giving you an error is that you are telling it to
expect a Vector of Strings. You happen to only have one, but a csv has
multiple lines. So that should be a Vector of Vectors of Strings.
The reason is says char is that it is assuming what is actually in the file
is a Vector of a List of Chars which also fits the pattern, because String
is a list of chars. Your original type would work with an input of
a,b,c
d,e,f
which is potentially valid, but for what you actually want, try this:
csvData <- DBL.readFile $ head args
case decode NoHeader csvData of
Left err -> putStrLn err
Right vec -> print (vec :: DV.Vector (DV.Vector String))
On Tue, May 6, 2014 at 2:29 PM, Ari King
I don't have pg installed so I can't run your code but I assume you are
breaking on the vector pattern matching. Pattern matching using : only works because the : operator is a constructor in lists.
:i (:) data [] a = ... | a : [a]
Remember that : is an infix operator, but it is comparable to Just, Left, or Right. You are trying to use a list constructor to pattern match on a vector which looks nothing like a list.
However you can do this sort of pattern matching by using vector's (or almost any other collection's) toList function:
hostaddr:port:dbname:username:password:rest = toList vec
Thanks for the clarification; I was under the impression that vectors were essentially re-implemented lists.
<- is a monadic binding. You use it when you are dealing with a datatype that happens to be an instance of Monad. It happens to be the only way to get anything out of an IO type, which is what you are using it for here. If something is just
using plain types, Int, String, etc, just use lets.
So, <- is more like extract (from IO type) and bind, but scoping wise is the same as let or where, correct?
Lastly, the code fails to read (see line 16 @ http://pastebin.com/R5MPNaHs) the provided file into a ByteString, ByteString readFile complains:
Failed reading: conversion error: expected Char, got "127.0.0.1"
The file contents are: 127.0.0.1,5432,sample,test_user,test_pwd
Am I misusing, ByteString's readFile? Thanks.
-Ari
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