Type checker's expected and inferred types (reformatted)

Isaac Dupree ml at isaac.cedarswampstudios.org
Sun Oct 25 17:23:51 EDT 2009

David Menendez wrote:
> On Sun, Oct 25, 2009 at 1:37 PM, Isaac Dupree
> <ml at isaac.cedarswampstudios.org> wrote:
>> David Menendez wrote:
>>> The expected type is what the context wants (it's *ex*ternal). The
>>> inferred type is what the expression itself has (it's *in*ternal).
>>> So inferring the type Maybe () for bar seems wrong.
>> well, maybe GHC just gets it wrong enough of the time, that I got confused.
>> Or maybe ... When there are bound variables interacting, on the inside and
>> outside, it gets confusing.
>> ghci:
>> Prelude> \x -> (3+x) + (length x)
>> <interactive>:1:15:
>>    Couldn't match expected type `[a]' against inferred type `Int'
>>    In the second argument of `(+)', namely `(length x)'
>>    In the expression: (3 + x) + (length x)
>>    In the expression: \ x -> (3 + x) + (length x)
>> Your explanation of "expected" and "inferred" could make sense to me if the
>> error message followed the "Couldn't match" line with, instead,
>>    "In the first argument of `length', namely `x'"
>> because 'length' gives the context of expected list-type, but we've found
>> out from elsewhere (a vague word) that 'x' needs to have type Int.
> This had me confused for a while, but I think I've worked out what's
> happening. (+) is polymorphic,   ...

Oh darn, it sounds like you're right. And polymorphism is so common.  I 
just came up with that example randomly as the first nontrivial 
type-error-with-a-lambda I could think of...

I wonder if it would help for the message to also output what it thinks 
the complete type of the function is (so far).

I wonder if it could look something like this:
Couldn't match expected type `[a]' against inferred type `Int'
In the second argument of `(+) :: [a] -> [a]',
namely `(length x) :: Int'

(always, at the risk/tradeoff of taking up space with lots of useless 
information. sadly. hmm.)

Ideally I would like GHC to pick a location for the error that's more 
intuitive, but that sounds like an awfully vague desire :-)


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