Instance of Read class in Haskell

Question

I was just starting having fun with haskell when I got stuck.

I'm trying to make my new data type (let's call it MyType) instance of the Read class. Mytype is a type constructor, so it takes another type as parameter. I wanted to write this kind of code

    instance (Read a) => Read (MyType a) where
        readsPrec _ r = [foo (read r :: a ), r]

but it gives me the following error

Could not deduce (Read a2) arising from a use of `read' from the context (Read a).

I thought that since a is Readable I could just infer it, but apparently I'm wrong. Any ideas?

EDIT: I have changed the previous code to

readsPrec _ r = [foo (read r :: a ), ""]

so if I type: read "myString" :: MyType a it works perfectly fine. Now I was hoping that if I would use read "myString" within a context, I shouldn't have to specify the type to read. But the problem is that with

bar (read myString) a

where bar:: MyType a -> a -> MyType a, I got Ambiguos variable type.

Is it possible to do something like that without getting that kind of error?

I hope it's clearer now, I'm trying to simplify the code but I hope I didn't omit anything crucial.

foo, it's actually a parser, it's a bit more complicated in my code I was trying to simplify here. Now it actually compile even without the ScopedTypeVariables, but as you said in the other post I probably misunderstood the way readPresc works because now I got Exception: Prelude.read: no parse When I try to read a string as MyType. — user1544128
– user1544128, Commented Nov 23, 2012 at 9:35
ok so in my case to be correct should be [foo (read r :: a ), ""], hope will be helpful to some other newbies :) — user1544128
– user1544128, Commented Nov 23, 2012 at 10:58
Sorry, but I find your edit confusing. Can you rephrase that as a question? — Joachim Breitner
– Joachim Breitner, Commented Nov 23, 2012 at 11:39

Joachim Breitner · Accepted Answer · 2012-11-23 09:01:11Z

6

The code actually typechecks if written as

instance (Read a) => Read (MyType a) where
        readsPrec _ r = [(foo (read r),r)]

if foo has type a -> MyType a. The compiler can figure out from the expected type signature of readsPrec that the call to foo ought to return a MyType a, and hence (by the type of foo), the expression read r ought to have type a.

But why does it fail when you annotate that with :: a? Because type variables are local to the type signature they appear in. So the a there is totally unrelated to the a in the instance heade and with read r :: a you are actually saying: The expression read r can have any arbitrary type. But an arbitrary type has no Read instance, hence the error message. In the message, the compiler renamed the inner a to a2 to avoid a name clash.

Your code can work as expected, though, if you add {-# LANGUAGE ScopedTypeVariables #-} to the module header. Now, the a in read r :: a refers to the type of a in the instance header and all goes well.

^{Note that you are not using readsPrec correctly, but I guess that is not part of the question.}

answered Nov 23, 2012 at 9:01

Joachim Breitner

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2 Comments

user1544128 Over a year ago

I added the ScopedTypeVariables and it actually compile, but when I try to read I got Exception: Prelude.read: no parse. Could you explain what do you mean about the fact that I'm not using readsPrec correctly? I thought I just needed to return a list with 1 tuple with MyType and the parsed String but apparently I was wrong again.

Joachim Breitner Over a year ago

You should not return the parsed String but rather what of the String is not parsed yet. Also, the list should contain all possible parses of the String (if there are several) and the empty list if there are none (and not an exception). In general, if you implement readsPrec, chances are hight that you should also use readsPrec for a.

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