Lispisms

Shai Berger https://bitbucket.org/shaib/lispism/

what is special about lisp

• invented - recursion, dynamic typing, garbage collection
• are there more gems to uncover?
• almost macros were considered and rejected in python
• MacroPy does some of it

dynamically scoped variables (DSV)

• for years, people ask for request object to be available globally
• answer: Globals are evil! toe code to system, value change unpredictably
• so use thread locals
• but there’s more to Evil than being harmful
• globals are really *@&$ ing convenient
• we prefer not to pass connections into every QuerySet
• settings can be accessed everywhere

What if we could have globals that do behave

• every change is temporary, applied only down the call chain - think with statement
• then each piece of code can be reasoned about

Local change is the norm in Lisp

• example, and equivalent in python
• but python likes mutables and objects - managing bindings is not enough
• obvious problem is that code inside can still change object state
• solution: make objects immutable
• what if we need to change just part of the object
• solution: special syntax for attribute change

lispism library

• special object called D

D.let(a=1, b=2):
    print D.a
    D.let(a=3, b=4):
        ...

Builtin containers are replaced by immutables - eg list -> tuple

Freezing

Limitations

• freezing can be worked around, but will avoid mistaken changes
• can’t control C extensions
• generators - yield inside let breaks the contract
• __dict__ access is a problem (plan to fix it)

Future

• threading
• use MacroPy macros for better sub-object control
• implement lisp restarts
  • separate What from How in error handling
  • code detects error handles it, but can’t make decision
  • higher code sets map of extension -> action label
  • low level code lists action labels, and actions for each case
  • intermediate code knows nothing