Haskish

The built-in library is intentionally minimal: building standard functions yourself is one of the best ways to learn functional programming. If you prefer, a richer set of Haskell Prelude functions is available by clicking here to load them into the function panel.

Arithmetic Functions

• a ^ b - Exponentiation: 2^10 → 1024
• mod a b - Modulo (remainder)
• div a b - Integer division
• min a b - Minimum of two values
• max a b - Maximum of two values

List Functions

Note: All list functions work on strings as well as lists, since strings are lists of characters (String = [Char]).

• head list - First element of a list
• tail list - All but the first element
• length list - Length of a list
• null list - Check if list is empty
• reverse list - Reverse a list
• take n list - Take first n elements
• drop n list - Drop first n elements

List Operators

• : - Cons (prepend element to list): 1:[2,3] → [1,2,3]
• ++ - Concatenate lists: [1,2]++[3,4] → [1,2,3,4]
• !! - Index into list: [10,20,30]!!1 → 20

Higher-Order Functions

• map fn list - Apply function to each element
  Example: map (*2) [1,2,3] → [2,4,6]
• filter predicate list - Filter list by predicate
  Example: filter (>5) [3,7,2,9] → [7,9]
• fold fn acc list - Reduce list with accumulator
  Example: fold (+) 0 [1,2,3] → 6

Note: fold is implemented as a left fold (same behaviour as foldl). foldr is also available.

Operator Sections

• (+1) - Partial application: map (+1) [1,2,3] → [2,3,4]
• (<10) - Comparison: filter (<10) [5,15,3] → [5,3]
• (*) - Binary operator as function: fold (*) 1 [2,3,4] → 24

List Comprehensions

• [expr | x <- list] - Map over list
  Example: [x*2 | x <- [1,2,3]] → [2,4,6]
• [expr | x <- list, guard] - Filter with guard
  Example: [x | x <- [1..10], x > 5] → [6,7,8,9,10]
• [expr | x <- list1, y <- list2] - Nested generators
  Example: [(x,y) | x <- [1,2], y <- ['a','b']] → [(1,'a'),(1,'b'),(2,'a'),(2,'b')]

Logic Functions

• not bool - Boolean negation

Character Functions

• ord char - Convert character to Unicode code point
• chr code - Convert Unicode code point to character
• show value - Convert any value to a string
  Example: show 42 → "42", show True → "True"
• putStr string - Print a string to the REPL without quotes
  Example: putStr "hello\nworld" → renders on two lines
• putStrLn string - Print a string followed by a newline
  Example: putStrLn "done" → done with trailing newline

String escape sequences: \n (newline), \t (tab), \\ (backslash), \" (double quote) are supported in string literals.

Control Flow

• if condition then expr1 else expr2 - Conditional expression (both branches required)
  Example: if x > 0 then "positive" else "non-positive"

Pattern Matching

• _ - Wildcard pattern (matches any value without binding)
  Example: getThird _ _ x = x

Language Features

• let ... in ... - Local bindings inside expressions, including multiline layout-sensitive forms
• case ... of - Pattern matching with constructors, tuples, lists, wildcards, and guards
• data Type = Ctor ... - Haskell-style data declarations (constructors work for values and pattern matching; no full type inference/checking)

Where Clauses

• where - Define local variables and functions scoped to the enclosing function
  Each binding on its own indented line; supports plain variables, local functions, and guarded local functions (see the Where Clauses example)

Other

• error message - Throw an error
• compose g f - Function composition (also g . f)
• True / False - Boolean values (capitalised)
• otherwise - Always true (for guards)

REPL Commands

• :help - Show all available REPL commands
• :show - Show all defined functions and variables
• :functions - List all user-defined functions
• :vars - List all user-defined variables
• :info functionName - Show definition(s) of a specific function
• :timeout 15000 - Set execution timeout for current session only (:timeout to view, :timeout reset for default)