"Simple Implicit Lambda", by Suzi & Martelli
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PEP: 312
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Title: Simple Implicit Lambda
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Version: $Revision$
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Last-Modified: $Date$
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Author: Roman Suzi <rnd at onego.ru>, Alex Martelli <aleax at aleax.it>
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Status: Draft
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Type: Standards Track
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Content-Type: text/plain
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Created: 11-Feb-2003
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Python-Version: 2.4
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Post-History:
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Abstract
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This PEP proposes to make argumentless lambda keyword optional in
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some cases where it is not grammatically ambiguous.
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Motivation
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Lambdas are useful for defining anonymous functions, e.g. for use
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as callbacks or (pseudo)-lazy evaluation schemes. Often, lambdas
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are not used when they would be appropriate, just because the keyword
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"lambda" makes code look complex. Omitting lambda in some special
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cases is possible, with small and backwards compatible changes to
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the grammar, and provides a cheap cure against such "lambdaphobia".
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Rationale
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Sometimes people do not use lambdas because they fear to introduce
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a term with a theory behind it. This proposal makes introducing
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argumentless lambdas easier, by omitting the "lambda" keyword.
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itself. Implementation can be done simply changing grammar so it
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lets the "lambda" keyword be implied in a few well-known cases.
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In particular, adding surrounding brackets lets you specify nullary
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lambda anywhere.
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Syntax
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An argumentless "lambda" keyword can be omitted in the following cases:
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* immediately after "=" in named parameter assignment or default value
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assignment;
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* immediately after "(" in any expression;
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* immediately after a "," in a function argument list;
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* immediately after a ":" in a dictionary literal; (not implemented)
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* in an assignment statement; (not implemented)
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Examples of Use
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1) Inline "if":
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def ifelse(cond, true_part, false_part):
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if cond:
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return true_part()
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else:
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return false_part()
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# old syntax:
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print ifelse(a < b, lambda:A, lambda:B)
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# new syntax:
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print ifelse(a < b, :A, :B)
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# parts A and B may require extensive processing, as in:
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print ifelse(a < b, :ext_proc1(A), :ext_proc2(B))
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2) Locking:
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def with(alock, acallable):
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alock.acquire()
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try:
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acallable()
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finally:
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alock.release()
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with(mylock, :x(y(), 23, z(), 'foo'))
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Implementation
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Implementation requires some tweaking of the Grammar/Grammar file
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in the Python sources, and some adjustment of Modules/parsermodule.c
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to make syntactic and pragmatic changes.
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(Some grammar/parser guru is needed to make a full implementation.)
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Here are the changes needed to Grammar to allow implicit lambda:
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varargslist: (fpdef ['=' imptest] ',')* ('*' NAME [',' '**'
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NAME] | '**' NAME) | fpdef ['=' imptest] (',' fpdef ['='
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imptest])* [',']
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imptest: test | implambdef
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atom: '(' [imptestlist] ')' | '[' [listmaker] ']' |
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'{' [dictmaker] '}' | '`' testlist1 '`' | NAME | NUMBER | STRING+
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implambdef: ':' test
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imptestlist: imptest (',' imptest)* [',']
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argument: [test '='] imptest
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Three new non-terminals are needed: imptest for the place where implicit
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lambda may occur, implambdef for the implicit lambda definition itself,
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imptestlist for a place where imptest's may occur.
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This implementation is not complete. First, because some files in Parser
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module need to be updated. Second, some additional places aren't
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implemented, see Syntax section above.
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Discussion
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This feature is not a high-visibility one (the only novel part is
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the absence of lambda). The feature is intended to make null-ary
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lambdas more appealing syntactically, to provide lazy evaluation
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of expressions in some simple cases. This proposal is not targeted
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at more advanced cases (demanding arguments for the lambda).
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There is an alternative proposition for implicit lambda: implicit
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lambda with unused arguments. In this case the function defined by
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such lambda can accept any parameters, i.e. be equivalent to:
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lambda *args: expr. This form would be more powerful. Grep in the
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standard library revealed that such lambdas are indeed in use.
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One more extension can provide a way to have a list of parameters
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passed to a function defined by implicit lambda. However, such
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parameters need some special name to be accessed and are unlikely
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to be included in the language. Possible local names for such
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parameters are: _, __args__, __. For example:
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reduce(:_[0] + _[1], [1,2,3], 0)
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reduce(:__[0] + __[1], [1,2,3], 0)
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reduce(:__args__[0] + __args__[1], [1,2,3], 0)
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These forms do not look very nice, and in the PEP author's opinion
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do not justify the removal of the lambda keyword in such cases.
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Credits
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The idea of dropping lambda was first coined by Paul Rubin at 08
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Feb 2003 16:39:30 -0800 in comp.lang.python while discussing the
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thread "For review: PEP 308 - If-then-else expression".
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Copyright
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This document has been placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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End:
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