Basics

1. Evaluation & REPL Errors

All valid Gleam syntax is evaluated cleanly.
Invalid Gleam syntax is reported by the REPL with the exact compiler error.
Errors are trapped gracefully protecting the VM from crashing.

gleam

Erlang/OTP 28 [erts-16.1.2] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Interactive Gleam (GSH 1.2.0) - press Ctrl+C to exit (type :h ENTER for help)
gsh(1)> 1
1 : Int
gsh(2)> 3.01 *. 3.14
9.4514 : Float
gsh(3)> 1 + "two"
error: Type mismatch
   ┌─ REPL:19:9
   │
19 │     1 + "two"
   │         ^^^^^

The + operator expects arguments of this type:

    Int

But this argument has this type:

    String

Hint: Strings can be joined using the `<>` operator.

2. Bindings & Shadowing

GSH is stateful.
When you bind a variable using let, gsh caches it in the shell-state.
Those bindings seamlessly cascade into your subsequent evaluations, and Gleam's native lexical shadowing works exactly as you would expect in a compiled file.

gleam

gsh(1)> let x = 10
10 : Int
gsh(2)> let x = x * 2
20 : Int
gsh(3)> x
20

Side effects are managed by wrapping these assignments in Erlang process-dictionary checks.
Side-effects execute exactly once, rather than re-firing every time the shell re-compiles the historical AST.

gleam

gsh(6)> let _ = io.println("Fired!")
Fired!
Nil
gsh(7)> echo 1
src/gsh_eval_7.gleam:30
1
1

3. Multiline Expressions & Top-Level Definitions

Unlike standard compiled Gleam, gsh allows you to define custom types and top-level functions directly in the prompt for rapid prototyping.
The pub modifier doesn't make any difference in the top-level.

gleam

gsh(1)> type Status {
...> Online
...> Offline
...> }
gsh(2)> pub fn check_status(s: Status) {
...> case s {
...> Online -> "All systems online"
...> Offline -> "Disconnected"
...> }
...> }
gsh(3)> check_status(Online)
"All systems online"
gsh(4)> check_status(Offline)
"Disconnected"

4. Type & Function Pruning

Old types & functions defined in the top-level when re-defined will be pruned.
Pruning is limited to fast prototyping! Changing the shape of the types or functions being chained with existing functions will result in broken shell-state till you fix the shape every where else!

gleam

gsh(1)> type Something {
...> A
...> B
...> }
gsh(2)> fn wibble(param: Something) {
...> case param {
...> B -> "hello"
...> _ -> "nah"
...> }
...> }
gsh(3)> wibble(A)
"nah"
gsh(4)> type Something {
...> Online
...> Offline
...> }
error: Unknown variable
   ┌─ REPL:18:1
   │
18 │ B -> "hello"
   │ ^

The custom type variant constructor `B` is not in scope here.
gsh(5)> fn wibble(param: Int) {
...> 2 + 4
...> }
gsh(6)> wibble(3)
6

5. Multi-line Support

gsh supports multi-line evaluations smartly based on Gleam's syntax.
If you made a mistake in the middle of a multi-line evaluation, you can press ctrl + x to abort it.

gleam

gsh(1)> fn wobble() {
...> "The
...> Quick
...> Brown
...> Fox
...> "
...> }
gsh(2)> wobble()
"The\nQuick\nBrown\nFox\n"
gsh(3)> type Struct {
...> Struct(
...> v1: Int,
...> v2: WrongType,
...> <something that won't compile!>
...> ^X
gsh(3)>