LangChain Expression Language (LCEL) basics

Introduction

LCEL helps you write simple expressions to control how LangChain works. It makes building chains easier and clearer.

You want to combine different steps in a LangChain workflow.

You need to create conditions or logic inside your chain.

You want to reuse small expressions for cleaner code.

You want to quickly test or change parts of your chain without rewriting code.

Syntax

LangChain

expression := value | variable | function_call | expression operator expression

value := number | string | boolean

operator := + | - | * | / | == | != | && | || | > | < | >= | <=

function_call := functionName(arg1, arg2, ...)

LCEL expressions look like simple math or logic statements.

You can use variables and call built-in functions inside expressions.

Examples

Adds two numbers and returns 8.

LangChain

5 + 3

Checks if the variable userInput equals the string "hello".

LangChain

userInput == "hello"

Calls a function to change userName to uppercase letters.

LangChain

toUpperCase(userName)

Returns true if score is greater than 10 and isActive is true.

LangChain

score > 10 && isActive

Sample Program

This example defines a simple chain with two steps. The first step checks if the user said "hello". The second step chooses a response based on that check. When userInput is "hello", it prints a friendly greeting.

LangChain

chain = {
  "steps": [
    {"name": "checkGreeting", "expression": "userInput == 'hello'"},
    {"name": "response", "expression": "'Hi there!' if checkGreeting else 'Please say hello.'"}
  ]
}

userInput = 'hello'
checkGreeting = eval(chain['steps'][0]['expression'])
result = eval(chain['steps'][1]['expression'])
print(result)

Important Notes

LCEL is designed to be easy to read and write, like simple math or logic.

Remember to use quotes around strings and proper operators for logic.

LCEL expressions run inside LangChain to control flow and data.

Summary

LCEL lets you write small expressions to control LangChain chains.

You can use variables, operators, and functions in LCEL.

It helps make your chains clearer and easier to manage.

Practice

1. What is the main purpose of LangChain Expression Language (LCEL)?

easy

A. To create full applications without coding

B. To write small expressions that control LangChain chains

C. To replace Python in LangChain completely

D. To design user interfaces for LangChain

Solution

1. Step 1: Understand LCEL's role

   LCEL is designed to write expressions that help control how LangChain chains behave.

2. Step 2: Compare options

   Only To write small expressions that control LangChain chains correctly describes LCEL's purpose; others describe unrelated tasks.

3. Final Answer:

   To write small expressions that control LangChain chains -> Option B

4. Quick Check:

   LCEL purpose = control chains with expressions [OK]

Hint: LCEL is for small expressions controlling chains [OK]

Common Mistakes:

• Thinking LCEL replaces full programming
• Confusing LCEL with UI design tools
• Assuming LCEL builds entire apps

2. Which of the following is the correct syntax to use a variable named input in an LCEL expression?

easy

A. {{input}}

B. {input}

C. $input

D. input

Solution

1. Step 1: Recall LCEL variable usage

   In LCEL, variables are used directly by their name without extra symbols.

2. Step 2: Evaluate options

   Only input uses the variable name plainly, which is correct syntax in LCEL.

3. Final Answer:

   input -> Option D

4. Quick Check:

   Variable usage = plain name [OK]

Hint: Use variable names directly without braces or symbols [OK]

Common Mistakes:

• Adding braces or dollar signs around variables
• Using double curly braces like in templates
• Confusing LCEL with other templating syntaxes

3. Given the LCEL expression input + ' world' where input is 'Hello', what is the output?

medium

A. 'input world'

B. 'Hello+ world'

C. 'Hello world'

D. Error: invalid operation

Solution

1. Step 1: Understand the expression

   The expression adds the string ' world' to the variable input which holds 'Hello'.

2. Step 2: Perform string concatenation

   Concatenating 'Hello' + ' world' results in 'Hello world'.

3. Final Answer:

   'Hello world' -> Option C

4. Quick Check:

   String concat = 'Hello world' [OK]

Hint: Adding strings concatenates them in LCEL [OK]

Common Mistakes:

• Treating + as a literal character
• Expecting variable name output instead of value
• Assuming syntax error on string addition

4. What is wrong with this LCEL expression: if input == 'yes' then 'ok' else 'no'?

medium

A. LCEL does not support if-then-else syntax like this

B. The variable name input is invalid

C. The strings should use double quotes instead of single quotes

D. The expression is correct and will work

Solution

1. Step 1: Check LCEL conditional syntax

   LCEL uses ternary-like syntax: condition ? true_value : false_value, not if-then-else.

2. Step 2: Analyze the given expression

   The expression uses if-then-else which is not valid in LCEL, causing a syntax error.

3. Final Answer:

   LCEL does not support if-then-else syntax like this -> Option A

4. Quick Check:

   Conditional syntax = ternary only [OK]

Hint: Use ternary ?: for conditions, not if-then-else [OK]

Common Mistakes:

• Using if-then-else like in other languages
• Assuming single quotes cause errors
• Thinking variable names are restricted

5. How would you write an LCEL expression to return the length of a variable text only if it is not empty, otherwise return 0?

hard

A. text != '' ? length(text) : 0

B. length(text) if text else 0

C. text ? length(text) : 0

D. if text != '' then length(text) else 0

Solution

1. Step 1: Understand LCEL conditional syntax

   LCEL uses ternary syntax: condition ? true_value : false_value.

2. Step 2: Check condition for non-empty string

   To check if text is not empty, use text != ''.

3. Step 3: Combine condition and function

   Use text != '' ? length(text) : 0 to return length if not empty, else 0.

4. Final Answer:

   text != '' ? length(text) : 0 -> Option A

5. Quick Check:

   Conditional length check = text != '' ? length(text) : 0 [OK]

Hint: Use ternary with condition text != '' for non-empty check [OK]

Common Mistakes:

• Using if-then-else syntax
• Assuming text alone is false if empty
• Using Python-style if expressions