Determining the type of an expression
Problem
How can I find out the type of an expression or a subexpression?
Solution
Type of expressions
You can start the interactive environment of Hugs or GHC (GHCi)
and use the :type
directive:
Prelude> :type map fromEnum "Text"
map fromEnum "Text" :: [Int]
If the expression is a top-level binding in a module,
you can use the :info
directive.
Prelude> :info map
map :: (a -> b) -> [a] -> [b]
Using :browse Modulename
you can show the types of all top-level variables.
This way you can write many functions in a module
without a type signature, find them out and add them afterwards.
But be warned that due to the Monomorphism restriction
the automatically infered signatures
are not always the ones you want.
Type of subexpressions
Say, you have the expression map f "Text" == [1,2,3]
and you want to find out of what type the f
must be.
This is also possible, just type
Prelude> :type \ f -> map f "Text" == [1,2,3]
\f -> map f "Text" == [1,2,3] :: Num a => (Char -> a) -> Bool
Here Bool
is the type of map f "Text" == [1,2,3]
and Num a => (Char -> a)
is the type of f
.
Now imagine, that the function f
cannot be chosen freely,
but is an instantiation of a polymorphic function.
E.g. of what concrete type is id
in map id "Text"
?
Prelude> :type \ f -> map (id `asTypeOf` f) "Text"
\f -> map (asTypeOf id f) "Text" :: (Char -> Char) -> [Char]
Using Data.Typeable
Say you'd like to catch a specific exception that is thrown, but you don't know which one it is. In the repl, you can catch SomeException and use Data.Typeable.typeOf
on the value inside SomeException:
λ> import Data.Typeable
λ> let catcher (SomeException e) = print (typeOf e)
λ> let failer = openFile "foo" WriteMode >> openFile "foo" WriteMode >> print "opened twice"
λ> catch failer catcher
IOException