Introduction to Scheme
Jump to navigation
Jump to search
When you run ./memcp, you will be dropped at a scheme shell like this:
memcp Copyright (C) 2023, 2024 Carl-Philip Hänsch
This program comes with ABSOLUTELY NO WARRANTY;
This is free software, and you are welcome to redistribute it
under certain conditions;
loading storage /tmp/x/system/bdf64a22-6315-463c-bbf7-329317ad50ed-id of type 10
loading storage /tmp/x/system/bdf64a22-6315-463c-bbf7-329317ad50ed-username of type 20
loading storage /tmp/x/system/bdf64a22-6315-463c-bbf7-329317ad50ed-password of type 20
Welcome to memcp
performing unit tests ...
finished unit tests
test result: 15/15
all tests succeeded.
Initializing SQL frontend
MySQL server listening on port 3307 (connect with `mysql -P 3307 -u root -p` using password 'admin')
listening on http://localhost:4321
Type (help) to show help
>
Scheme is a functional programming language and a subset of LISP. Every expression is either a primitive value or a list.
To understand the semantics of Scheme, take a look at the following Scheme REPL session:
> 12 = 12 > "hi" = "hi" > (+ 1 2) = 3 > (+ 1 2 3) = 6 > (+ 2 (* 2 2)) = 6 > (+ 2 (* 2 4)) = 10 > (concat "Hello" "World") = "HelloWorld"
Syntax
Function Call
A function call has the following format:
(functionname param1 param2 param3 ...)
Function calls start with (, contain one function and zero or more parameters separated by space .
Some basic functions are + - * / print concat. For more consult the manual with (help).
Literals
The following literals are allowed:
- Number Literals:
1 2 3 56 7.5 4.3e20 -6.123e-3 - String Literals:
"Hello World" "First Line\nSecond Line" "he said: \"what?\" and smiled" - Function names:
print - Symbol literals
'print - List literals
'(1 2 3) '("a" "b" "c") - Associative Array literals
'("key" "value" "size" 45 "name" "Peter" "sublist" '(1 2 3))- associative arrays are lists with key value pairs flattened down to a one dimensional list of an even number of items. - Lambdas:
(lambda (param1 param2 ...) body)
Lambda Functions
Scheme allows for creating lambda functions which enclose their own scope:
(define decrease-by-one (lambda (number) (- number 1)))
The function can now be called from scheme:
> (decrease-by-one 5) = 4
Lambda functions can be passed to other functions as values. They enclose your original scope, so you can use variables from the outside inside your function.
Deeper Topics into Scheme
You can deep-dive into the following topcis:
Further Help and Documentation of all Functions
If you type (help) into the console, you will get the following overview:
> (help)
Available scm functions:
-- SCM Builtins --
quote: returns a symbol or list without evaluating it
eval: executes the given scheme program in the current environment
optimize: optimize the given scheme program
if: checks a condition and then conditionally evaluates code branches; there might be multiple condition+true-branch clauses
and: returns true if all conditions evaluate to true
or: returns true if at least one condition evaluates to true
coalesce: returns the first value that has a non-zero value
coalesceNil: returns the first value that has a non-nil value
define: defines or sets a variable in the current environment
set: defines or sets a variable in the current environment
error: halts the whole execution thread and throws an error message
try: tries to execute a function and returns its result. In case of a failure, the error is fed to the second function and its result value will be used
apply: runs the function with its arguments
apply_assoc: runs the function with its arguments but arguments is a assoc list
symbol: returns a symbol built from that string
list: returns a list containing the parameters as alements
string: converts the given value into string
match: takes a value evaluates the branch that first matches the given pattern
lambda: returns a function (func) constructed from the given code
begin: creates a own variable scope, evaluates all sub expressions and returns the result of the last one
parallel: executes all parameters in parallel and returns nil if they are finished
source: annotates the node with filename and line information for better backtraces
-- Arithmetic / Logic --
number?: tells if the value is a number
+: adds two or more numbers
-: subtracts two or more numbers from the first one
*: multiplies two or more numbers
/: divides two or more numbers from the first one
<=: compares two numbers
<: compares two numbers
>: compares two numbers
>=: compares two numbers
equal?: deep-compares two values of the same type
equal??: performs a sloppy equality check on primitive values (number, string, bool. nil), strings are compared case insensitive
!: negates the boolean value
not: negates the boolean value
nil?: returns true if value is nil
min: returns the smallest value
max: returns the highest value
floor: rounds the number down
ceil: rounds the number up
round: rounds the number
-- Strings --
string?: tells if the value is a string
concat: concatenates stringable values and returns a string
substr: returns a substring
simplify: turns a stringable input value in the easiest-most value (e.g. turn strings into numbers if they are numeric
strlen: returns the length of a string
strlike: matches the string against a wildcard pattern (SQL compliant)
toLower: turns a string into lower case
toUpper: turns a string into upper case
replace: replaces all occurances in a string with another string
split: splits a string using a separator or space
htmlentities: escapes the string for use in HTML
urlencode: encodes a string according to URI coding schema
urldecode: decodes a string according to URI coding schema
-- Lists --
append: appends items to a list and return the extended list.
append_unique: appends items to a list but only if they are new.
cons: constructs a list from a head and a tail list
car: extracts the head of a list
cdr: extracts the tail of a list
merge: flattens a list of lists into a list containing all the subitems. If one parameter is given, it is a list of lists that is flattened. If multiple parameters are given, they are treated as lists that will be merged into one
merge_unique: flattens a list of lists into a list containing all the subitems. Duplicates are filtered out.
has?: checks if a list has a certain item (equal?)
filter: returns a list that only contains elements that pass the filter function
map: returns a list that contains the results of a map function that is applied to the list
mapIndex: returns a list that contains the results of a map function that is applied to the list
reduce: returns a list that contains the result of a map function
produce: returns a list that contains produced items - it works like for(state = startstate, condition(state), state = iterator(state)) {yield state}
produceN: returns a list with numbers from 0..n-1
list?: checks if a value is a list
contains?: checks if a value is in a list; uses the equal?? operator
-- Associative Lists / Dictionaries --
filter_assoc: returns a filtered dictionary according to a filter function
map_assoc: returns a mapped dictionary according to a map function
reduce_assoc: reduces a dictionary according to a reduce function
has_assoc?: checks if a dictionary has a key present
extract_assoc: applies a function (key value) on the dictionary and returns the results as a flat list
set_assoc: returns a dictionary where a single value has been changed.
merge_assoc: returns a dictionary where all keys from dict1 and all keys from dict2 are present.
-- Parsers --
parser: creates a parser
-- Sync --
newsession: Creates a new session which is a threadsafe key-value store represented as a function that can be either called as a getter (session key) or setter (session key value) or list all keys with (session)
once: Creates a function wrapper that you can call multiple times but only gets executed once. The result value is cached and returned on a second call. You can add parameters to that resulting function that will be passed to the first run of the wrapped function.
mutex: Creates a mutex. The return value is a function that takes one parameter which is a parameterless function. The mutex is guaranteed that all calls to that mutex get serialized.
-- IO --
print: Prints values to stdout (only in IO environment)
help: Lists all functions or print help for a specific function
import: Imports a file .scm file into current namespace
load: Loads a file and returns the string
serve: Opens a HTTP server at a given port
mysql: Imports a file .scm file into current namespace
password: Hashes a password with sha1 (for mysql user authentication)
-- Storage --
scan: does an unordered parallel filter-map-reduce pass on a single table and returns the reduced result
scan_order: does an ordered parallel filter and serial map-reduce pass on a single table and returns the reduced result
createdatabase: creates a new database
dropdatabase: creates a new database
createtable: creates a new database
createcolumn: creates a new column in table
shardcolumn: tells us how it would partition a column according to their values. Returns a list of pivot elements.
altertable: alters a table
droptable: removes a table
insert: inserts a new dataset into table
stat: return memory statistics
show: show databases/tables/columns
rebuild: rebuilds all main storages and returns the amount of time it took
loadCSV: loads a CSV file into a table and returns the amount of time it took.
loadJSON: loads a .jsonl file from disk into a database and returns the amount of time it took.
settings: reads or writes a global settings value. This modifies your data/settings.json.
get further information by typing (help "functionname") to get more info