Re: A new view of guards

[Hans Aberg]

At 09:15 97-04-28, Simon L Peyton Jones wrote:

>A proposal
>~~~~~~~~~~~
>The proposal I want to make is simple:
>
>        Instead of being a boolean expression,
>        a guard is a list of qualifiers,
>        exactly as in a list comprehension.
>
>        That is, the only syntax change is to replace
>        "exp" by "quals" in the syntax of guarded equations.
>
>Here is how I would write clunky:
>
>  clunky env var1 var1 | Just val1 <- lookup env var1,
>                         Just val2 <- lookup env var2
>                       = val1 + val2
>  ...other equations for clunky...

  Are you not just asking for another equivalence relation, "patterns
match"? Call it, say, "==="; then you could write

  clunky env var1 var1 | Just val1 === lookup env var1 &&
                         Just val2 === lookup env var2
                       = val1 + val2
  ...other equations for clunky...

but also combine with other Boolean expressions.

  Hans Aberg