4 ms·

Have they ever fixed the problem with let not being implemented (which was the case when I looked at it a couple of years ago). Lisp-like languages without let

Have they ever fixed the problem with let not being implemented (which was the case when I looked at it a couple of years ago). Lisp-like languages without let lose a lot of the inherent power of lisps.

rcarmo 5y ago
It's sort of back, implemented as a macro. I've yet to go back to Hy (I left because of the removal of let, plus other things I found missing), but I try it every now and then and the last time I did the namespacing was still a tad weird.
- gilch 5y ago
  Have you tried Hissp yet?
EnderShadow8 5y ago
This might be interesting https://pypi.org/project/scoping/ https://pypi.org/project/scoping/
Kodiologist 5y ago
It's been implemented, found to be subtly broken, and re-implemented a number of times. Ultimately trying to impose a different model of scoping on top of Python's scoping is living in a state of sin; thus the difficulties. The status quo is an apparently working `let` due to a truly heroic effort by Sunjay Cauligi ( https://github.com/hylang/hy/pull/2125 https://github.com/hylang/hy/pull/2125 ). You're encouraged to try to find bugs in it. In truth, I've never understood the fuss about `let`. It's taken a huge amount of Hy development effort, but it adds very little. Python's native scoping works well in the large majority of cases, and in tricky cases, I'd rather use `nonlocal` or `global` explicitly so I'm sure what's happening at the level of Python semantics. I suspect that people who heard that previous versions of Hy were missing `let` assumed that some other feature (such as nested lexical scopes) was also missing that has actually always been there.
- contravariant 5y ago
  What problems did you encoutner implementing 'let'? Shouldn't it be equivalent to lambda expressions somehow?
  - Kodiologist 5y ago
    In some other Lisps, yes, but Python has its own idea of how function scoping works that generally isn't what you'd want with `let`. For example, in (let [a 1] (defn foo [] (setv a 2)) (foo) (print a)) you probably `2` to be printed, but the seeming equivalent Python code a = 1 def foo(): a = 2 foo() print(a) prints `1`. Python interprets the assignment as declaring a new local variable. Frankly, Python got scoping wrong (in its quest to eliminate declarations), but here we are. So `let` needs to work around this somehow. The current implementation implicitly adds `global a` or `nonlocal a` as appropriate. (Actually, the name `a` is also mangled so that any references to `a` outside the `let` refer to a different variable.)
    - ashtonbaker 5y ago
      this made me wonder - I know I can use global variables inside a function definition: a = 1 def foo(): x = a foo() so if this 'a' is a new variable, what happens when the name collides with a global variable? a = 1 def foo(): a = a foo() > UnboundLocalError: local variable 'a' referenced before assignment yuck!
      - all2 5y ago
        I'm not sure this is so much of a problem? Changing what you name the variable fixes the issue. Python is generally quite good at having a verbose option to express things, so I'm surprised there's not an explicit way to reference the enclosing scope from an object.
      - vegai_ 5y ago
        Is there any non-horror piece of code where supporting this latter case would be useful?
mark_l_watson 5y ago
They are working on improving it. I wrote a short little book on Hy and I initially used the “let” macro. I stopped using it because in general Hy code can be automatically translated to very readable Python code, but using the “let” macro generates ugly Python code.

rcarmo 5y ago

It's sort of back, implemented as a macro. I've yet to go back to Hy (I left because of the removal of let, plus other things I found missing), but I try it every now and then and the last time I did the namespacing was still a tad weird.

gilch 5y ago
Have you tried Hissp yet?

EnderShadow8 5y ago

This might be interesting https://pypi.org/project/scoping/ https://pypi.org/project/scoping/

Kodiologist 5y ago

It's been implemented, found to be subtly broken, and re-implemented a number of times. Ultimately trying to impose a different model of scoping on top of Python's scoping is living in a state of sin; thus the difficulties. The status quo is an apparently working `let` due to a truly heroic effort by Sunjay Cauligi ( https://github.com/hylang/hy/pull/2125 https://github.com/hylang/hy/pull/2125 ). You're encouraged to try to find bugs in it. In truth, I've never understood the fuss about `let`. It's taken a huge amount of Hy development effort, but it adds very little. Python's native scoping works well in the large majority of cases, and in tricky cases, I'd rather use `nonlocal` or `global` explicitly so I'm sure what's happening at the level of Python semantics. I suspect that people who heard that previous versions of Hy were missing `let` assumed that some other feature (such as nested lexical scopes) was also missing that has actually always been there.

contravariant 5y ago
What problems did you encoutner implementing 'let'? Shouldn't it be equivalent to lambda expressions somehow?
- Kodiologist 5y ago
  In some other Lisps, yes, but Python has its own idea of how function scoping works that generally isn't what you'd want with `let`. For example, in (let [a 1] (defn foo [] (setv a 2)) (foo) (print a)) you probably `2` to be printed, but the seeming equivalent Python code a = 1 def foo(): a = 2 foo() print(a) prints `1`. Python interprets the assignment as declaring a new local variable. Frankly, Python got scoping wrong (in its quest to eliminate declarations), but here we are. So `let` needs to work around this somehow. The current implementation implicitly adds `global a` or `nonlocal a` as appropriate. (Actually, the name `a` is also mangled so that any references to `a` outside the `let` refer to a different variable.)
  - ashtonbaker 5y ago
    this made me wonder - I know I can use global variables inside a function definition: a = 1 def foo(): x = a foo() so if this 'a' is a new variable, what happens when the name collides with a global variable? a = 1 def foo(): a = a foo() > UnboundLocalError: local variable 'a' referenced before assignment yuck!
    - all2 5y ago
      I'm not sure this is so much of a problem? Changing what you name the variable fixes the issue. Python is generally quite good at having a verbose option to express things, so I'm surprised there's not an explicit way to reference the enclosing scope from an object.
    - vegai_ 5y ago
      Is there any non-horror piece of code where supporting this latter case would be useful?

mark_l_watson 5y ago

They are working on improving it. I wrote a short little book on Hy and I initially used the “let” macro. I stopped using it because in general Hy code can be automatically translated to very readable Python code, but using the “let” macro generates ugly Python code.