Enter the hole where it doesn't work
I have the following
readStatement :: String -> IO [Transaction]
readStatement path = do
csvData <- readFile path
return $ catMaybes (map _ (splitOn "\r" csvData))
The GHC will find the next hole type [Char] -> Maybe Transaction
. Now if I give a name to this hole and go to the where clause
readStatement :: String -> IO [Transaction]
readStatement path = do
csvData <- readFile path
return $ catMaybes (map process (splitOn "\r" csvData))
where process = _
GHC cannot find the newline type, but gives:
Found hole ‘_’ with type: t
Where: ‘t’ is a rigid type variable bound by
the inferred type of process :: t at Statement.hs:63:11
Relevant bindings include
process :: t (bound at Statement.hs:63:11)
path :: String (bound at Statement.hs:60:15)
readStatement :: String -> IO [Transaction]
(bound at Statement.hs:60:1)
In the expression: _
In an equation for ‘process’: process = _
In an equation for ‘readStatement’:
readStatement path
= do { csvData <- readFile path;
return $ catMaybes (map process (splitOn "\r" csvData)) }
where
process = _
The code is strictly equivalent, and how is it that type inference does not give the same result? (I am using GHC 7.8.3)
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In the following code
let x = something
in f x
type inference x = something
is ignored f
. That is, the type something
is inferred, generalized to polymorphic types, and x
gets this type. This can assign a x
type that is more general than the type required for f
.
More specifically, in
let x = \y->y
in x "hello"
we output polymorphic binding x :: forall a. a->a
. Please note that it String
is not displayed yet.
I think the GHC prints information about the holes of the type at that time, only after doing type inference in the definition let
. In the example posted
do one
two
where process = _
we are outputting _ :: t
without going through the context and are not printing this type. If you prefer a more specialized (monomorphic) type, try instead
(\process -> do
one
two
) _
We do not use let
or above where
, so only monomorphic types will be inferred.
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