Fun With Vars

FUN with Vars
Bay Area Clojure Meetup
Aug 6, 2009

Kyle Oba
Runa Employee #8
@mudphone
kyleoba@gmail

Tuesday, August 11, 2009

agenda

# => ikouCLOJURE
• demonstration of the early stages of an online api lookup tool

how routing works in WEBBING
• why not start with a macro?
• how it turned out to be a Var problem

READER => EVALUATION => VARS
• in which you should remember THREE things
1. var is a special form
2. this is possible: symbol map Var binding value

3. if a symbol is NOT RESOLVED to a value ON EVALUATION => BOOM!

RECUR


var = special

sym - Var - value
agenda unresolv’d sym eval boom

# => ikouCLOJURE
• demonstration of the early stages of an online api lookup tool

how routing works in WEBBING
• why not start with a macro?
• how it turned out to be a Var problem

READER => EVALUATION => VARS
• in which you should remember THREE things
1. var is a special form
2. this is possible: symbol map Var binding value

3. if a symbol is NOT RESOLVED to a value ON EVALUATION => BOOM!

RECUR


ikouCLOJURE : routing

(def routes-map {
...

"/doc/get" ikou-doc/get-doc
"/docs/ﬁnd" ikou-doc/get-ﬁnd-doc

"/docs/ns" ikou-doc/docs-for-ns
"/names/ns" ikou-doc/names-for-ns

...
})


ikouCLOJURE

sort of demonstration


ikouCLOJURE : routing

(def routes-map {
"/doc/get" ikou-doc/get-doc
})

FIRST TRY: a macro (why not?)

(defmacro get-doc [thing]
`(with-out-str
(doc ~thing)))

get-doc WORKS at the REPL

but, it doesn’t EVALUATE as part of the routes-map
=> java.lang.Exception: Can't take value of a macro
at clojure.lang.Compiler.analyzeSymbol(Compiler.java:4684)


ikouCLOJURE : to be absolutely clear about that macro thing

(defmacro m-get-doc [thing]
`(with-out-str
(doc ~thing)))

(defn get-doc-w [x]
(m-get-doc x))

this WON’T EVALUATE

it throws a
java.lang.Exception: Unable to resolve var: x in this context
[Thrown class clojure.lang.Compiler$CompilerException]


ikouCLOJURE : to be absolutely clear that error

even this WON’T EVALUATE

(defn bogus-doc [x]
(doc x))

it throws a

ask folks to remember, we got an ERROR here

ikouCLOJURE : unresolvable Var error

why are we getting this error?



Clojure.org: a small vocabulary lesson

DATA STRUCTURES

READER

COMPILER

EVALUATION

SYMBOL

VAR


Clojure.org: http://clojure.org/reader

CLOJURE DATA STRUCTURES

“Clojure is a homoiconic language, which is a fancy term describing the fact that
Clojure programs are represented by Clojure data structures.”

“Clojure is defined in terms of the evaluation of data structures and not in terms of
the syntax of character streams/files.”

“That said, most Clojure programs begin life as text files, and it is the task of the
reader to parse the text and produce the data structure the compiler will see. This
is not merely a phase of the compiler. The reader, and the Clojure data
representations, have utility on their own...”

WTF? AWESOME, right?

Since itʼs just Clojure data structures, programs can create programs
Clojure programs could begin life as anything that leads to these data structures.

ninjas

REALULTIMATEPOWER.NET

this is why macros are possible
this is why clojure (and lisps) have parens

Clojure.org: http://clojure.org/reader

SRSLY. WHAT DID THAT MEAN?

“One might say the reader has syntax
deﬁned in terms of characters,

and the Clojure language has syntax
deﬁned in terms of symbols, lists, vectors, maps etc.”

text -> READER -> clojure data structures -> COMPILER -> EVALUATION

reader reads charaters (as forms)
compiler reads clojure language as clojure data structures, which are the objects returned by the reader

Clojure.org: http://clojure.org/Evaluation

EVALUATION

“Evaluation can occur in many contexts:
Interactively, in the REPL
On a sequence of forms read from a stream, via load or load-ﬁle
Programmatically, via eval”

text -> READER -> clojure data structures -> COMPILER -> EVALUATION


var = special

sym - Var - value
Clojure.org: http://clojure.org/Evaluation unresolv’d sym eval boom

EVALUATION

“ Clojure programs are composed of expressions.
Every form not handled specially by a special form
or macro is considered by the compiler to be an expression, which is evaluated to
yield a value...

...a single object is considered by the compiler, evaluated, and its result returned.
If an expression needs to be compiled, it will be. There is no separate compilation
step, nor any need to worry that a function you have deﬁned is being interpreted.
Clojure has no interpreter.”

THING TO REMEMBER #1: there are these “special forms”

var = special

sym - Var - value
Clojure.org: http://clojure.org/Evaluation unresolv’d sym eval boom

EVALUATION

SYMBOL Resolution
is namespace qualified? -> binding of named global var
is package qualified? -> Java class
else, in order...
1. is special form? -> special form stuff
2. maps to class in current ns? -> Java class
3. binds locally? -> value of local binding
4. maps to Var? -> value of binding of the Var
5. error

If you remember, we were trying to figure out why we had that error...

THING #2: if symbol maps to Var, then binding of Var is the evaluated value
THING #3: if symbol is not evaluated to a value, this causes an error

back to the error

so, we had that error...

even this WON’T EVALUATE

(defn bogus-doc [x]
(doc x))

it throws a

WHY?


here’s a hint

“doc” is a macro

which expands like so

user> (macroexpand-1 '(doc map))
(clojure.core/print-doc (var map))

and “print-doc” works like this

user> (print-doc #'map)
-------------------------
clojure.core/map
([f coll] [f c1 c2] [f c1 c2 c3] [f c1 c2 c3 & colls])
Returns a lazy sequence ...


back to the error

so, our offending fn is expands to

(defn bogus-doc [x]
(print-doc (var x)))

and it throws a

STILL WHY?


back to the error

now that we know more about symbol resolution and evaluation

let’s take a closer look


so maybe the var special form in bogus-doc is causing the problem

var = special

sym - Var - value
back to the error unresolv’d sym eval boom

real quick-like...

what does “def” do?

Var without binding
user=> (def x)
#'user/x
user=> x
java.lang.IllegalStateException: Var user/x is unbound.

Var with binding
user=> (def x 1)
#'user/x
user=> x
1

what then, is the var special form resolving x to (in bogus-doc)?

var = special

sym - Var - value
back to the error unresolv’d sym eval boom

so, it’s the var special form

when “defn bogus-doc” is evaluated

what does (var x) refer to?

(defn bogus-doc [x]
(doc x))

aka

(defn bogus-doc [x]
(print-doc (var x)))

answer: nothing

answer: nothing
hence the error

var = special

sym - Var - value
an example unresolv’d sym eval boom

you can get my-doc to evaluate properly,
if you declare y ﬁrst

user> (declare y)
#'user/y

user> (defn my-doc [y]
(doc y))
#'user/my-doc

but, what’s this?

user> (my-doc map)
-------------------------
user/y
nil
nil
nil

the doc info for “map” is not correct, it appears to be the doc info for user/y

var = special

sym - Var - value
what happened to y? unresolv’d sym eval boom

remember

doc is a macro

user> (macroexpand-1 '(doc ﬁlter))
(clojure.core/print-doc (var ﬁlter))

var is a special form

so...

• doc expands to (print-doc (var y)) at evaluation
• my-doc's print-doc argument is the Var #'user/y
• the "argument y" ISN’T USED


var = special

sym - Var - value
another example unresolv’d sym eval boom

another way to say that (var y) isn’t resolving to the fn’s arg

user> (defn verbose-doc [y]
(doc y)
(println y))
#'user/verbose-doc

user> (verbose-doc map)
-------------------------
user/y
nil
nil
#<core$map__4564 clojure.core$map__4564@20cc1fc4>
nil


RECUR : ikouCLOJURE

so, how do we ﬁx our original problem?

I wanted to get the doc output for an input string.

youʼve kicked that dead horse enough

RECUR : ikouCLOJURE

this works... and you don’t need a macro

(defn get-doc [str-name]
(with-out-str
(print-doc (resolve (symbol str-name)))))


var = special

sym - Var - value
RECUR : ikouCLOJURE unresolv’d sym eval boom

why not use "doc" ?

doc is a macro that syntactically replaces
the form with a (var y) special form

when evaluated
(var y) will cause an error if it doesn’t resolve to a value


ﬁnally

an interesting point

• "doc" info is just meta-data
• but where? on the Var, not on the object
• that's why print-doc takes a Var


finally

an example

metadata sits on the var

user> (meta (var map))

{:ns #<Namespace clojure.core>, :name map, :file "clojure/core.clj", :line
1588, :arglists ([f coll] [f c1 c2] [f c1 c2 c3] [f c1 c2 c3 & colls]), :doc "Returns a
lazy sequence consisting of the result of applying f to then set of first items of
each coll, followed by applying f to the setn of second items in each coll, until
any one of the colls isn exhausted. Any remaining items in other colls are
ignored. Functionn f should accept number-of-colls arguments."}


var = special

sym - Var - value
finally unresolv’d sym eval boom

if that made sense to you
then this might too

user> (declare y)
user> (defn huh [y]
(doc y)
(binding [map y]
(map "hello")
(meta #'map)))
user> (huh println)
-------------------------
user/y
nil ...
hello
{:ns #<Namespace clojure.core>, :name map, :file "clojure/core.clj", :line
1588, :arglists ([f coll] [f c1 c2] [f c1 c2 c3] [f c1 c2 c3 & colls]), :doc "Returns a lazy
sequence consisting of the result of applying f to then set of first items of each
coll, followed by applying f to the setn ...}


thank you any questions?


Fun With Vars

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