AILANG Debug

Fix common AILANG errors quickly.

Quick Reference

Error	Cause	Fix
undefined variable: print	Not in entry module	Use entry module or import std/io (print)
undefined variable: println	Wrong function name	Use print not println
undefined variable: map	Not imported	import std/list (map) or write recursive
No instance for Num[string]	print(42)	Use print(show(42))
expected }, got let	Missing semicolon	Add ; between statements
unexpected token: for	No loops in AILANG	Use recursion instead
unexpected token: in	No for x in xs	Use match xs { ... }

Decision Tree

Error message?
│
├─ "undefined variable: X"
│   └─ Is X a builtin?
│       ├─ Yes → Import it: ailang builtins list | grep X
│       └─ No → Check spelling, define it
│
├─ "expected }, got ..."
│   └─ Missing semicolon between statements
│       Fix: let x = 1; let y = 2; x + y
│
├─ "No instance for Num[string]"
│   └─ Passing number to string function
│       Fix: print(show(42)) not print(42)
│
├─ "unexpected token: for/while/in"
│   └─ AILANG has no loops!
│       Fix: Use recursion with match
│
└─ Parse error with braces
    └─ Unmatched { } or missing expression
        Fix: Check all blocks are closed

Common Fixes

1. Missing Semicolons

-- WRONG
export func main() -> () ! {IO} {
  let x = 10
  let y = 20
  print(show(x + y))
}

-- CORRECT (semicolons between statements)
export func main() -> () ! {IO} {
  let x = 10;
  let y = 20;
  print(show(x + y))
}

2. Print Needs String

-- WRONG: print expects string
print(42)

-- CORRECT: convert with show()
print(show(42))

3. No Loops - Use Recursion

-- WRONG: no for loops
for i in range(5) { print(show(i)) }

-- CORRECT: recursive function
export func printRange(n: int) -> () ! {IO} {
  if n <= 0 then () else {
    print(show(n));
    printRange(n - 1)
  }
}

4. Import Standard Library

-- WRONG: map not in scope
let doubled = map(\x. x * 2, nums)

-- CORRECT: import from std/list
import std/list (map)
let doubled = map(\x. x * 2, nums)

-- OR: write it yourself (recursion)
export func myMap[a,b](f: func(a) -> b, xs: [a]) -> [b] {
  match xs {
    [] => [],
    hd :: tl => f(hd) :: myMap(f, tl)
  }
}

Debugging Workflow

1. Type-check first (faster feedback):

   ailang check file.ail
   

2. Read error location - line:column tells you where

3. Check the pattern above for your error type

4. Use REPL for quick tests:

   ailang repl
   > show(42)
   > 1 + 2
   > :type \x. x * 2
   

5. List builtins to find imports (CLI is source of truth):

   # SOURCE OF TRUTH: Full documentation with examples
   ailang builtins list --verbose --by-module
   
   # Search for specific function with full docs
   ailang builtins list --verbose | grep -A 10 "map"
   
   # See a specific module's functions
   ailang builtins list --verbose --by-module | grep -A 30 "std/list"
   

Resources

Always prefer CLI commands (ailang prompt, ailang builtins list --verbose) over static docs - they're always up-to-date.

See resources/error_catalog.md for additional error patterns.