Simple `Read` instance incorrectly `read`s-CodePudding

Why does this Read instance parse inconsistently?

import qualified Data.List as List

data Foo = Foo

instance Show Foo where
  show _ = "Foo"

instance Read Foo where
  readsPrec _ s = case List.stripPrefix "Foo" s of
    Just rest -> [(Foo, rest)]
    Nothing -> []

This is expected:

Test> reads "" :: [(Foo, String)]
[]

This is unexpected:

Test> read "" :: Foo
Foo

I would expect it to throw.

CodePudding user response：

The problem isn't in read, but in show. Your show implementation doesn't force the input value of type Foo, because it returns "Foo" unconditionally, without even matching a constructor. If you use the derived Show instance, or write its equivalent by hand:

instance Show Foo where
  show Foo = "Foo"

then you will get the expected error when you try to parse a malformed string, because evaluating show will actually require parsing the input string.