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PSCi "PureScript Interactive" is the REPL for PureScript. It is often a good way to explore your code, do some basic testing, or to get a feel for a new library.

Getting Started

Use Spago to configure and start PSCi:

$ npm install -g spago       # Install Spago

$ cd my-project              # Enter an empty folder

$ spago init                 # Initialize a Spago environment

$ spago repl                 # Fire up the interpreter psci

...
PSCi, version 0.15.0
Type :? for help

import Prelude

> "hello"
"hello"

> import Prelude

> 1 + 2 * 3
7

> import Effect.Console

> log "print this to the screen"
print this to the screen
unit

For more information about getting started with a development environment, see this guide.

Basic usage

Type expressions into the REPL to have them evaluated:

> 3 + 4
7

Introduce bindings with =:

> x = 4
> x + 9
13

You can also define data types, type classes, and type class instances (you may need to use multi-line mode for this):

> data Season = Spring | Summer | Autumn | Winter
> class Shout a where shout :: a -> a
> instance shoutString :: Shout String where shout s = s <> "!"

Paste mode (previously --multi-line-mode)

Enter :paste (or :pa) to enter multi-line (or "paste") mode. Terminate it with Control-D key.

> import Prelude
> :paste
… add :: Int -> Int -> Int
… add = \x y -> x + y
> add 10 20
> (^D)
30

Demo

purescript-psci-support

$ spago repl

PSCi requires the `purescript-psci-support` package to be installed.
You can install it using Spago by adding `"psci-support"` to the list of dependencies in `spago.dhall`.

For help getting started, visit http://wiki.purescript.org/PSCi

The PureScript compiler suite (i.e. the executable purs), unlike most compilers, does not ship with a standard library. In PureScript, even Prelude is a normal module, just like any other. Consequentially, purs repl requires a specific library to be installed in order to be able to evaluate terms in the REPL.

purescript-psci-support defines the Eval type class for this purpose. Instances of Eval are provided for Showable types, and for Effect, so that we can evaluate actions in the REPL.

PSCi Without Spago

PSCi can be run directly, by specifying a list of PureScript source files as globs:

purs repl 'src/**/*.purs' 'path/to/packages/**/*.purs'

This expects you to have downloaded the PureScript sources for your dependencies (include psci-support) under the path path/to/packages.

Note the single quotes—the purescript compiler itself knows how to expand globs (*) and recursive globs (**), single quotes prevent your shell from expanding them. (Bash for example doesn’t have recursive globbing enabled by default; don’t forget the single quotes.)