There should be a way to mark blocks as pure.

[paulstansifer]

The following code has to name two functions.

pure fn hash(x: @str) -> uint { str::hash(*x) }
pure fn eq(x: @str, y: @str) -> uint { str::eq(*x,*y) }
ret hashmap(hash, eq);

It'd be nice if we could write:

ret hashmap(pure {|x: @str|str::hash(*x)}, pure {|x,y|str::eq(*x,*y)});

...or even (with type inference, if possible):

ret hashmap({|x: @str|str::hash(*x)}, {|x,y|str::eq(*x,*y)});

[jld]

It sounds as if you want those functions to be pure, not the expressions that compute the function values. I have the same problem in something I'm writing: exposing access to a piece of unique data within a data structure via a callback, which has to be pure to be able to safely borrow the argument. And the only way to create a value to inhabit a pure fn type is with a named fn statement, which doesn't close over its environment, which leaves me back in C-land passing around explicit environment structs (and not being able to use do-notation). So that's kind of annoying.

I tried modifying the parser to accept pure fn expressions, but that ran into the assert at typeck/check.rs line 197, and at this point I do not even pretend to understand what's going on here.

[graydon]

purity is going away, closing this as WONTFIX