How to give infixities to operators in lambda?

Question

For example, this does not type check

\cons nil -> 5 `cons` 3 `cons` nil

nor does this

\(#) -> 5 # 3 # nil

Although both of these do

\cons nil -> 5 `cons` nil
\(#) nil -> 5 # nil

Is there a way to assign infixites to operators in lambdas. I tried

infixr 5 #
foo = \(#) nil -> 5 # 3 # nil

which gives an error for no definition of # and

foo = \(infixr 5 #) nil -> 5 # 3 # nil

which is just a syntax error.

What can I do?

score 16 · Answer 1 · answered Dec 05 '15 at 18:45

16

Fixity declarations can be local but must accompany definitions, so you'd have to write something like

foo cons nil = 'a' # 'b' # nil
  where (#) = cons
        infixr 5 #

or

foo = \cons nil -> let (#) = cons; infixr 5 # in 'a' # 'b' # nil

etc.

answered Dec 05 '15 at 18:45

Reid Barton

4

Nice indeed! Is this part of Haskell98, Haskell2010, or just GHC? – leftaroundabout Dec 05 '15 at 20:33
1

I especially like the let version. – PyRulez Dec 05 '15 at 23:16
1

It is Haskell 98. The syntax of let and where involves "decls", which includes type signatures, fixity declarations and function and pattern bindings. – Reid Barton Dec 06 '15 at 03:17
please don't write the `let` version – jberryman Dec 06 '15 at 03:56

1 Answers1