Haskell language report: reservedid definition

Question

Good, that's why I played all day with the Haskell Language Report (2010) and found all kinds of “interesting” edges. Material that, in your opinion, should be a mistake, but which is actually allowed.

In particular, consider the following excerpt from a lexical syntax reference (section 10.2):

reservedid → case | class | data | default | deriving | do | else | foreign | if | import | in | infix | infixl | | infixr | instance | let | module | newtype | of | then | type | where | _ 

You may see qualified in this list & hellip; But he is not. (Not as or hiding , for that matter.)

Now I'm curious and hellip; Is this a random observation in the Report? Or is it a deliberate design decision?

The GHC seems to at least follow the letter of the specification, as it will gladly allow you to define a variable whose name is essentially qualified . Strange, but true. Thus, it seems that this name is only "special" in one context. On the contrary, you can never name a module variable. We could make this word special only at the beginning of the & hellip; but we did not.

Answer 1

qualified , as and hiding are found only in certain places as keywords, so they can be used as variable names.

• qualified only happens after import .
• as occurs only after import qualified PACKAGE
• hiding only happens after import [qualified] PACKAGE [as NAME]

I understand what you mean by pointing out that module may be one of these keywords related to the placement, but it seems counterintuitive to call anything module . Maybe the previous version of Haskell allows several modules in a single file?

You can say the same for qualified and hiding , but I often used as , for example:

 zip [] _ = [] zip _ [] = [] zip (a:as) (b:bs) = (a, b) : zip as bs 

So, with that in mind, I think this is really a design decision that I think can include many languages ​​*.

It may be worth noting that deriving may be one of these words because it always follows the data declaration, as well as foreign , since it is reserved only with the FFI extension.

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* Especially javascript, with it is a huge list of mostly meaningless reserved words !

Answer 2

First, the keywords “how,” “hide,” and “qualify” were not in the original Haskell definition. When they were added, they were not made into reserved words for backward compatibility. Adding reserved words is a good way to break code. It was a very deliberate decision.

Secondly, as far as I know, Haskell does not prohibit multiple modules per file. The spec just doesn't talk about files. (But I do not know any implementation that allows this.)